Preheat the oven to 350F. Line baking sheets with parchment paper or silicone baking mats.
Cream the butter and sugar on medium-high speed until light and fluffy.
Mix in the egg.
Mix in the cocoa powder at low speed until well blended.
Add the flour, salt and baking powder to the bowl and mix on low speed just until incorporated.
Make round cookie balls about 1 1/2-2 inch in diameter.
Place ball onto cookie sheet and use palm of hand to flatten ball. Make a shallow well in the center of the cookie and place a piece of chocolate in the center.
Place marshmallow on top, pressing it in slightly. Make sure your well is wide enough to hold the large marshmallow so it doesn't spread off the cookie when baking. Repeat with remaining cookies, make sure to place each cookie about 2 inches apart.
Bake about 10 minutes until done.
Serve after cookies as cooled or else the melted chocolate center may burn your mouth.
