1. In a medium bowl, sift together flour, cocoa powder, baking powder, and salt. Set aside.
2. In the bowl of a stand mixer or using a hand mixer, beat sugars and oil for 2-3 minutes, until fluffy. Add eggs one at a time, beating to incorporate. Stir in vanilla, then fold in flour mixture, combining until all the flour is incorporated and the batter is thick and sticky. 
3. Cover the bowl tightly with plastic wrap and refrigerate at least two hours, up to 24 (I usually let it chill overnight).
4. When you’re ready to bake, preheat oven to 350 degrees and line two baking sheets with parchment paper. Place powdered sugar in a medium bowl. 
5. Remove the batter from the fridge and using a small cookie scoop or spoon, scoop up one-inch balls of dough, rolling them to shape with your hands. Roll in powdered sugar and place on parchment-lined baking sheets, two inches apart. Bake for 10 minutes, then carefully remove from oven and place a marshmallow slice atop each cookie. Return to oven and bake 2 minutes more, until marshmallows begin to puff up. 
6. Cool for a few minutes on the baking sheet to let the cookies set, then transfer to a baking rack to finish cooling. If desired, dust the tops with a little cocoa powder, cinnamon, or crushed candy canes. You can eat them warm but I think they’re at their chewy, chocolate-y best at room temperature. 

Store leftovers in an airtight container, separating layers of cookies with wax paper or nonstick foil (or the marshmallows will stick).