Line a large baking sheet with parchment paper. Set aside
In a large bowl, whisk together flour, dark cocoa powder, black cocoa powder, baking powder and salt. Set aside.
Beat butter and brown sugar with an electric mixer until light and creamy. Scrape down the sides of the bowl with a spatula as needed.
Add egg and vanilla then beat until incorporated.
Reduce mixer speed to low, add dry ingredients to wet and mix until just combined.
Stir in chocolate chips with a spoon until evenly distributed. Avoid overmixing.
Scoop dough into rounded balls with a large cookie scoop.
Using your thumb, make large indent into the top of each dough ball. Insert a halved marshmallow then pinch dough up and around marshmallow.
Place cookie dough balls on your parchment-lined cookie sheet, spacing them 2-inches apart.
Preheat oven to 350 degrees.
While oven is preheating, place cookie sheet in the freezer so dough gets cold. This quick chilling will help prevent cookies from spreading while baking.
Bake cookies for 9 to 10 minutes or until the edges are set but the centers are still soft. When in doubt, underbake. Overbaked cookies will be dry.
Allow cookies to cool on the baking sheet for 5 minutes before removing.
Store cookies in an airtight container at room temperature up to 3 days.