Measure out the chocolate chips: reserve 1/3 cup for topping and use the remaining 2/3 cup for the dough.

In a small microwave-safe bowl, combine 3 tablespoons unsalted butter and the 2/3 cup chocolate chips. Microwave for 30 seconds, stir, then continue microwaving in 15-second bursts, stirring after each, until smooth. Set aside to cool for about 5 minutes (cool enough that it will not cook the egg).

In a large bowl, stir together 1/3 cup + 1 heaping tablespoon (85 g) brown sugar, 1 large egg, and 3/4 teaspoon vanilla extract until just combined.

Stir the cooled chocolate-butter mixture into the egg mixture until evenly combined.

In a separate bowl, whisk together 1/2 cup (70 g) all-purpose flour, 1 tablespoon + 1 teaspoon (10 g) cocoa powder, 1/2 teaspoon baking powder, and 1/8 teaspoon salt.

Add the dry ingredients to the wet ingredients and mix until just combined and no large streaks of flour remain. Do not overmix.

Cover the dough and chill in the refrigerator for 45 minutes, or until firm enough to handle.

While the dough chills (or during the last 10 minutes), preheat the oven to 325°F. Line two baking sheets with parchment paper or lightly grease them.

Roll the chilled dough into 1-inch balls and place them on the prepared baking sheets about 2 inches apart.

Bake the cookies for 7 minutes.

Remove the sheets from the oven. Press 2–3 of the reserved chocolate chips into the center of each cookie, then top each cookie with one mini marshmallow (24 total). Press the marshmallow gently so it contacts the chips.

Return the cookies to the oven and bake an additional 4 minutes.

Remove the baking sheets from the oven. If using, sprinkle finely chopped chocolate over the cookies while they are still hot so the chocolate melts slightly. Let the cookies cool on the baking sheets for about 5 minutes before transferring to a wire rack to cool completely.