Preheat oven to 350°F. Mix flour, cocoa powder, baking soda and salt in medium bowl. Set aside. Beat butter and sugars in large bowl with electric mixer on medium speed until light and fluffy. Add eggs and vanilla; mix well. Gradually beat in flour mixture on low speed until well mixed. Stir in chocolate chips and Sixlets.: As the oven warms, you will notice the kitchen begin to take on a faint, warm heat that primes your baking surface. Preheating ensures even baking and correct spread. A common mistake is placing cookies into an oven that is not fully heated, which can cause uneven texture and underbaked centers. Check that racks are centered so heat circulates evenly, and if your oven has hotspots, rotate the baking sheet halfway through baking for uniform color and doneness.
Drop by rounded tablespoons about 2 inches apart onto baking sheets sprayed with no stick cooking spray.: Right away you will smell the earthy aroma of cocoa powder mingling with the faint bite of baking soda, which is normal. Sifting or whisking helps remove lumps and ensures an even distribution of leavening, preventing streaks of raw cocoa in the dough. A troubleshooting tip, if you notice clumps of cocoa try pressing them through a fine mesh or whisking longer, because unincorporated lumps lead to uneven pockets of dry powder in the cookie.
Bake 8 to 10 minutes or just until cookies are set. Cool on baking sheets 2 minutes. Remove to wire racks; cool completely.: As you cream, you will see the butter and sugars transform from pale to lighter and slightly aerated. This creates tiny air pockets that contribute to tender texture and slight lift. If your butter is too cold it will not cream properly, if it is too warm the mixture will be greasy. The right texture looks smooth and ribbon like when the beater is lifted, so stop when you reach that stage to avoid overbeating which can deflate the air and yield dense cookies.
Add eggs and vanilla; mix well.: When the eggs and vanilla go in, the batter will gloss over and smell more aromatic. Eggs help emulsify the mixture, creating cohesion. If eggs are cold they can cause the batter to seize; bring them to room temperature for a smoother blend. If you see curdling, beat a spoonful of dry ingredients in to rescue the texture and keep going.
Gradually beat in flour mixture on low speed until well mixed.: Adding the dry mix slowly keeps the dough from becoming overworked, preserving tenderness. On low speed, you will notice the dough darken and thicken, and the beaters may slow as the mass becomes more substantial. Overmixing activates gluten and yields a tougher cookie, so stop when there are no visible streaks of flour. If you still find pockets of dry mix, fold by hand with a spatula to finish gently.
Stir in chocolate chips and Sixlets.: As you fold in the chocolate chips and Sixlets , you will see glossy pockets and bright candy peeking through the dough. Distribute them evenly so every cookie has both melty chocolate and candy crunch. If you dump them in and overmix, the candies can bleed color into the dough, so fold just until combined. A common issue is clumping of chips in one zone, so scrape the bowl and fold from the bottom to ensure even spread.
Drop by rounded tablespoons about 2 inches apart onto baking sheets sprayed with no stick cooking spray.: Hearing the slight thud as each scoop hits the sheet is oddly satisfying, and spacing is key so cookies have room to spread and not fuse. Use a cookie scoop for uniform size; inconsistent scoops produce uneven bake times. If cookies bake into each other, reduce size or increase spacing, and consider using parchment for easier release and cleanup.
Bake 8 to 10 minutes or just until cookies are set.: During baking, you will see the edges take on a faint sheen and the centers look slightly glossy. The aroma of cocoa and melting chocolate will intensify and waft through your kitchen. Remove at the visual cue of set centers, because overbaking can dry them out. If you find your cookies consistently overcook, lower the time or temperature slightly and test a small batch to dial in the perfect window.
Cool on baking sheets 2 minutes.: Letting the cookies rest on the sheet allows residual heat to finish the cooking and helps them hold their shape when moved. You will notice they firm up a touch and the melted chips settle. Resist the urge to transfer immediately, because they can tear or flatten if moved while too soft. If they seem overly soft after two minutes, give them an extra minute before transferring.
Remove to wire racks; cool completely.: As the cookies cool on wire racks, air circulates beneath them, preventing soggy bottoms and preserving crisp edges. You will see the candy set and the chocolate return to a more stable state. For storage, once completely cool, place them in an airtight container so they maintain texture. A common mistake is stacking hot cookies which can trap steam and soften them, so always ensure complete cooling first.