Whisk together flour, cocoa, baking powder, and salt in a medium bowl; set aside. Melt chocolate in a bowl in the microwave using 30-second increments, stirring once or twice, until smooth. Set bowl aside.: The dry mix will smell faintly of cocoa and have a dusty, velvety texture, which readies it to absorb the wet ingredients for a fudgy consistency. Doing this ahead ensures even distribution so pockets of baking powder do not create odd bumps, and the cocoa disperses for uniform color. If you skip thorough whisking you might find streaks of cocoa or uneven rise in baked cookies.
Beat eggs and vanilla lightly with a fork, sprinkle the coffee powder over to dissolve, and set aside. I do this in a one-cup Pyrex measuring cup.: As the semisweet chocolate softens you will see glossy ribbons form, and stirring will encourage a silky, homogeneous sheen; the smell becomes intensely chocolatey and warm. Microwaving short bursts prevents scorching, and stirring evens out hot spots. If you overheat, the chocolate can seize into a grainy mass, so stop heating when a few unmelted pieces remain and stir until smooth.
In the bowl of a standing mixer fitted with the paddle attachment, beat butter just until smooth and creamy.: This mixture will smell aromatic from the vanilla and slightly bitter from the dissolved coffee, adding depth. Dissolving the coffee into the eggs avoids gritty specks in the dough and helps the flavor integrate. If the coffee does not dissolve well, your cookies may have uneven flavor pockets, so ensure you whisk it until gone.
Beat in sugars until combined. Reduce speed to low and gradually beat in egg mixture until incorporated.: The butter will soften to a pale, satiny texture and a faint dairy aroma will develop, which helps aeration. Stopping when it is simply smooth preserves the right amount of air for texture. If you overbeat to the point of becoming too light, the cookies can spread too much when baked.
Add chocolate and beat until just combined. Scrape the bottom and sides of the bowl with a spatula. With the mixer at low speed, add the flour mixture and mix until combined. Do not overbeat. Mix in chocolate chips.: As the brown sugar and granulated sugar incorporate, the mixture will smell caramel like and feel slightly grainy then smooth; this creates the moisture balance and sweetness backbone. Proper creaming helps dissolve the sugars, preventing grainy pockets in the dough. Under mixing can leave undissolved sugar, which makes gritty bites, while over mixing can create too much incorporation of air.
If your dough is not fudge-like and scoopable, cover it with plastic and let rest for about 30 minutes. I did not need to do this step.: The batter will glossy slightly and smell richer as the egg and vanilla meld in, creating a cohesive emulsion that supports the melted chocolate . Adding slowly prevents curdling and ensures a smooth batter. If you add the eggs too fast the mixture can split, leading to an uneven texture.
Preheat oven to 350º. Line cookie sheets with parchment paper. Leaving about 1 1/2-inches between each ball, scoop dough using a medium cookie disher to form heaping tablespoon balls of dough.: The melted semisweet chocolate will cool the batter and create a dense, luxurious texture; visually the mix turns darker and thicker. Scraping the bottom and sides with a spatula ensures no streaks of flour remain. Overmixing here will incorporate too much air and change the tender, fudgy result, so stop when streaks disappear.
Roll each dough ball between your hands to make them nice and round. Press in a few extra chocolate chips to garnish, if desired.: The dough should come together into a thick, scoopable mass, with the flour no longer visible. This step sets the cookie structure, and mixing just until combined prevents tough cookies from overdeveloped gluten. A common error is overmixing which yields dense, chewy cookies rather than tender ones.
Bake cookies until edges have just begun to set but centers are still very soft about 10 minutes.: The semisweet chocolate chips will stud the dough, giving contrasting pockets of melty chocolate and small bursts of sweetness. Folding them in by hand prevents beating air into the dough and keeps the chips intact. Using a mixer on high can crush chips and cause them to melt too much into the batter.
Cool cookies on sheets for about 10 minutes, slide parchment with cookies onto a wire rack, and cool to room temperature.: Resting chills the dough slightly, allowing fats to firm and hydration to even out, which makes it easier to scoop and yields more uniform cookies. The texture will go from glossy to slightly firmer, and you will smell deeper chocolate notes as it sets. If your dough is already scoopable do not force a long chill, as extended refrigeration can dry the surface and alter bake time.
Preheat oven to 350º: The oven will come up to a steady heat that encourages the edges to set while the centers remain soft; you may notice a warm, ambient scent of heat when it reaches temperature. A fully preheated oven ensures consistent baking and predictable spread. Baking in an underheated oven will cause uneven cook and excessive spreading, so always preheat fully.
Line cookie sheets with parchment paper: Parchment provides an even, nonstick surface that encourages gentle browning and easy removal, and you will notice the bottoms turning a tender golden rather than sticking. It also protects against hot spots on the pan. Skipping parchment can cause cookies to stick and break when transferred.
Leaving about 1 1/2 inches between each ball, scoop dough using a medium cookie disher to form heaping tablespoon balls of dough: Spacing allows for even expansion, and the cookie disher produces uniform sizes so they bake at the same rate. You should hear a slight thud as the scooper releases the dough into the pan, and the balls will be glossy and compact. Crowding the pan will yield merged cookies and inconsistent centers.
Roll each dough ball between your hands to make them nice and round: Rolling smooths the surface for prettier domes and seals chocolate chips into the interior, producing a uniform crackle as they bake. If you press a few extra chocolate chips on top, they will melt into a pretty garnish. If your hands are too warm the dough will soften excessively, so cool your palms or chill briefly to maintain shape.
Bake cookies until edges have just begun to set but centers are still very soft about 10 minutes: As the cookies bake you will see the edges take on a firmer, slightly satin sheen while centers remain domed and tender, and the room will fill with a warm chocolate aroma. Removing them while centers are soft ensures a chewy, fudgy interior once cooled. Leaving them too long will produce drier, firmer cookies rather than that desirable soft center.
Cool cookies on sheets for about 10 minutes, slide parchment with cookies onto a wire rack, and cool to room temperature: During the rest they finish setting; you will notice the centers firming slightly and the bottoms becoming perfectly set. Sliding the parchment preserves shape and prevents breakage during transfer. If you try to move them too soon they will fall apart, so patience here protects the texture.