Step 1

Add a few inches of water to a saucepan and heat over low heat until simmering. Add the chopped chocolate to a heatproof bowl and set over the saucepan. Let melt, 3 to 4 minutes, then stir until smooth. Remove the bowl from the pan and let cool slightly.

Step 2

In a medium bowl, whisk together the flour, hot cocoa mix, espresso powder, baking powder, baking soda, and salt to combine and get rid of any lumps.

Step 3

In a large bowl, beat the butter and sugar with an electric 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.

Step 4

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.

Step 5

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 to the cookie, 2 to 3 minutes more. Let cool 5 minutes on the pans; remove to racks to cool completely.