Preheat the oven to 350°F and line a large baking sheet with parchment paper.
In a small bowl, mix together the cocoa powder, all-purpose flour, baking powder, and salt. Set aside.
Melt together the butter and 1/2 cup (90g) chocolate chips either in a double boiler or in the microwave in 30-second increments. Let cool.
Using a stand mixer or electric hand mixer (or some good muscle strength!) whisk together the sugar, egg, and vanilla extract. For 1-2 minutes until pale yellow. Slowly pour in the butter and chocolate mixture and mix until combined.
Fold in the dry ingredients and remaining chocolate chips (1/2 cup - 90g). Scoop approximately 1 3/4 TBSP of dough onto the baking sheet, spacing approximately 2 inches apart. Optional: for a smoother cookie, you can roll each dough ball in your hands. Gently press a thumb or teaspoon into the center of the cookie and press in the halved marshmallow, cut side down, into the cookie. The divit doesn't have to be too deep, you just want to be sure there is dough around the cookie so the marshmallow doesn't melt out. If omitting the marshmallow, I recommend topping with several chocolate chips, pressing the dough down slightly.
Bake for 8-10 minutes until edges are set and marshmallow begins to turn golden brown. I like to take them out right at 8 minutes so they remain fudgy. Let cool on the pan for about 2 minutes then transfer to a wire cooling rack.
For added effect, use a creme brulee torch to roast the marshmallows after baking.
Store in an air-tight container at room temperature.