½cupunsweetened cocoa powder, plus extra for dusting
1cup(2 sticks) cold, unsalted butter, cubed
2 ½cupssemi-sweet chocolate chips
16large marshmallows, cut in half
Preheat the oven to 350 F and line a cookie sheet with parchment paper.
In a medium bowl, whisk together the flour, cocoa, salt, baking powder, and espresso. Set aside.
In a separate bowl with an electric mixer, combine the butter and sugar. Beat on medium-high speed for a 2 to 3 minutes, until light and fluffy. Add eggs one at a time, scraping down the bowl as needed. Add the cocoa-flour mixture to the bowl and mix on low speed just until incorporated. Using a wood spoon or rubber spatula, stir in the chocolate chips.
Scoop a heaping tablespoon of dough, roll into a ball and place on the cookie sheet. Using the palm of your hand, slightly flatten the ball. Repeat with remaining dough placing the dough balls an inch from each other on the cookie sheet.
Bake cookies for 12 minutes, or until the edges are set, but the center isn't fully baked. Remove the sheet from the oven and place a marshmallow half on each cookie. Return the cookies to the oven to bake for an additional 2 to 4 minutes, or until the marshmallows are melting and starting to brown.
Let cool on the cookie sheets 5-10 minutes, then transfer to a wire rack to cool completely.