3 envelopes hot cocoa mix without marshmallows (about 1.38 ounces each)
1 tsp. instant espresso powder
1 tsp. baking powder
1/2 tsp. baking soda
1/4 tsp. kosher salt
2 sticks salted butter, at room temperature
1 3/4 c. sugar
2 large eggs
2 tsp. vanilla extract
1 1/2 c. white chocolate chips
24 marshmallows, halved crosswise with kitchen shears
Put the chopped chocolate in a heatproof bowl and set over a saucepan filled with a few inches of simmering water over low heat. Let melt, 3 to 4 minutes, then stir until smooth. Remove the bowl from the pan and let cool slightly.
Combine the flour, hot cocoa mix, espresso powder, baking powder, baking soda and salt in a medium bowl and whisk to get rid of any lumps.
Combine the butter and sugar in a large bowl and beat with a mixer on medium-high speed until light and smooth, 2 to 3 minutes. Add the eggs, one at a time, and beat until combined. Beat in the vanilla. With the mixer on low speed, gradually add the melted chocolate. Increase the speed to medium high and beat until smooth and light, about 1 minute. With the mixer on low speed, add the flour mixture and beat until just combined, 30 seconds to 1 minute. Stir in the white chocolate chips. Cover the dough and refrigerate until it just begins to firm up, about 30 minutes.
Preheat the oven to 350˚F. Line 2 baking sheets with parchment paper. Scoop the dough into balls (about 1 tablespoon each) and place on the baking sheets, about 2 inches apart. Bake until the edges just begin to set, 9 to 10 minutes.
Remove from the oven and place a marshmallow half on the center of each cookie, cut-side down. Continue baking until the edges of the cookies are set and the marshmallows are stuck on, 2 to 3 minutes more. Let cool for 5 minutes on the pans; remove to racks to cool completely.