From My Bowl

Prep: Preheat the oven to 350F 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.
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.
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.
Roll: Divide the dough into 12 equal balls, roughly 3 tablespoons each. Place each cookie dough ball 2” 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.
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.
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!)
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.