Preparation Steps

• Brown the butter in a light-colored saucepan over medium heat, stirring, until the milk solids turn toasty and the butter smells nutty, 3–5 minutes. Scrape into a mixing bowl and cool until just warm to the touch.
• While the butter cools, whisk together the flour, cocoa powder, milk powder, espresso powder, baking soda, and salt in a separate bowl.
• Heat the oven to 325°F (165°C). Line two baking sheets with parchment paper.
• Whisk the brown sugar and granulated sugar into the warm browned butter until glossy. Add the egg and vanilla; whisk vigorously for 30–45 seconds until slightly thickened and cohesive.
• Add the dry mixture in two additions, folding just until no dry streaks remain. Stir in the chocolate chips. Rest the dough for 15 minutes to hydrate (it will firm up slightly).
• Scoop the dough into 1 1/2-tablespoon portions (about a medium cookie scoop) and arrange 2 inches apart. Gently flatten the tops to about 3/4-inch thick.
• Bake one sheet at a time for 9 minutes, rotating the pan at the 7-minute mark. Remove the pan, use the back of a spoon to press a shallow well in the center of each cookie, then nestle a marshmallow half (cut side down) into each well.
• Return to the oven and bake 3–4 minutes more, until the marshmallows are puffed and the cookie edges are set. For a toastier marshmallow, switch to broil for 20–30 seconds—watch closely.
• Cool the cookies on the sheet for 10 minutes, then transfer to a rack to cool completely (the marshmallow will settle slightly, creating a perfect landing spot for chocolate).
• Melt the chopped dark chocolate with the neutral oil using a double boiler (or microwave at 60% power in 20–30 second bursts), stirring until smooth and fluid.
• Spoon or pipe about 1 to 2 teaspoons of melted chocolate over each marshmallow, nudging it to the edges to form a shiny cap. Let the chocolate set at room temperature, or refrigerate the tray for 10 minutes to speed it up.