Prep: Preheat the oven to 350 F and grease or line a baking sheet with parchment paper. Combine the ground flax with the water in a small bowl and mix well; let sit for 5 minutes, to thicken.: Preheat the oven to 350 F and grease or line a baking sheet with parchment paper : As the oven warms you will notice a rising warmth that promises browning and set in the cookie edges, and the gentle hum of the oven tells you it is ready. Preparing the baking sheet with parchment paper prevents sticking and makes cleanup effortless, and greasing is an option if you prefer a slightly crisper underside. A common mistake is not preheating fully, which leads to uneven spread and texture, so wait until the oven reaches the full temperature before baking. Visually you want the oven at a steady heat so the cookies puff then set, not collapse.
Cream the Butter: Add the butter, brown sugar, and cane sugar to a large bowl and use a hand mixer (or bowl of a stand mixer) to cream them together until smooth, 2 to 3 minutes. Add the thickened flax seed and vanilla to the butter mixture. Mix again until combined.: After stirring, the mixture will become glossy and slightly gelatinous, a sign the flaxseed has hydrated and is ready to act as a binder. This creates a sticky, egg like texture that helps the dough hold together and contributes to tenderness. If you skip the rest time the binding will be weaker and the dough may crumble, so be patient. The smell is mild and nutty, nothing overpowering, but you will feel the change in viscosity as it thickens.
Make the Dough: Add the flour to the mixing bowl and sift the cocoa powder on top to prevent clumps. Add in cornstarch, baking soda, and salt to the bowl and mix well with a spatula. Add 1/2 cup of the chopped chocolate to the bowl and fold into the dough.: Add the butter, brown sugar, and cane sugar to a large bowl and use a hand mixer or stand mixer to cream them together until smooth, 2 to 3 minutes : As you beat, the mixture will lighten in color and become airy, with the butter and sugars forming tiny pockets of air that help the cookies rise and have a tender crumb. Scrape the bowl sides to ensure even creaming, and then add the thickened flaxseed and vanilla , mixing until integrated. One common error is undercreaming which yields dense cookies, so take the time to reach a fluffy consistency. The aroma becomes sweeter and more buttery as the sugar dissolves into the fat.
Roll: Divide the dough into 12 equal balls, roughly 3 tablespoons each. Place each cookie dough ball 2 inches apart on the baking sheet, then top with the remaining 1/4 cup (~40 g) of chocolate, pressing it into the top of the ball slightly so it sticks. Do not flatten the cookies on the baking sheet as they will spread when they cook.: Add the flour to the mixing bowl and sift the cocoa powder on top to prevent clumps : Sifting the cocoa powder directly over the flour helps distribute it evenly and avoids dark streaks. Add cornstarch , baking soda , and salt , then fold gently with a spatula until mostly combined before adding 1/2 cup of the chopped chocolate . Folding minimizes gluten development, keeping the dough tender, and the batter will feel thick and slightly tacky. A frequent mistake is overmixing after the flour goes in, which toughens the cookie; stop as soon as dry streaks disappear. Visually, the dough becomes uniformly cocoa colored with flecks of chocolate.
Bake: Bake for 7 minutes, until the cookies have spread and are puffy. Remove the cookies from the oven and quickly press in 3-4 mini marshmallows per cookie; it’s ok if they flatten slightly as you decorate. Return the cookies to the oven and bake for an additional 2 minutes, to set the marshmallows.: Divide the dough into 12 equal balls, roughly 3 tablespoons each : As you roll, the dough will warm slightly and become even more pliable, and you will see soft lumps that hold shape but will spread. Placing them 2 inches apart allows room for spreading, and topping each ball with the reserved 1/4 cup of chopped chocolate so it adheres to the surface creates a glossy finish after baking. Avoid flattening them on the sheet because they need to dome to achieve the right texture; pressing too much early will make them thin and crispy instead of soft and puffy. The cookie dough should look slightly rounded and dense.
Cool: Remove the cookies from the oven and let sit on the baking sheet for 10 to 15 minutes, to allow them to set. Use a spatula to transfer the cookies to a wire rack and let them cool completely (or enjoy warm!).: Bake for 7 minutes, until the cookies have spread and are puffy : You will hear no crackle, but visually the cookies will expand and their surfaces will set while the centers remain soft. The edges may begin to hint at firmness, and the aroma of cocoa and caramelized sugars will bloom through your kitchen. After the initial bake, quickly press 3 to 4 mini marshmallows into each cookie and return to the oven for an additional 2 minutes to set the marshmallows without burning them. The mistake to avoid is overbaking in either phase, which dries the cookie and makes the marshmallows hard instead of pillowy.
Serve & Store: Let the chocolate fully cool and solidify before storing. Store cookies in an airtight container at room temperature for up to 4 days, or in the freezer for up to one month.: Remove the cookies from the oven and let sit on the baking sheet for 10 to 15 minutes, to allow them to set : Letting the cookies rest on the warm sheet allows residual heat to finish cooking the center gently, and you will notice the chocolate on top begin to solidify into glossy patches. Using a spatula to transfer them to a wire rack prevents them from steaming on the hot sheet, preserving texture. A common pitfall is moving them too soon, which can cause them to break apart while still soft. The cooled cookies should hold their shape and have a tender interior.
Serve & Store: Let the chocolate fully cool and solidify before storing : Once cooled, place the cookies in an airtight container at room temperature for up to 4 days, or freeze them for up to one month. Proper storage preserves the contrast between the soft center and the slightly firmer exterior. The most frequent storage mistake is stacking warm cookies which leads to sogginess, so always cool completely before layering. When serving, slightly warmed cookies bring back that freshly baked gooeyness, while chilled ones offer a firmer bite.