Melt semisweet chocolate using a double boiler, stirring until smooth.

Sift together flour, cocoa powder, baking powder, and salt in a bowl, then set aside.

In a separate bowl, cream the unsalted butter with brown sugar and white sugar until light and fluffy.

Beat in eggs one at a time, then add coffee powder and vanilla extract, blending well.

Stir in the melted chocolate until fully combined.

Gradually add the sifted dry ingredients to the wet ingredients, mixing until just combined.

Fold in the mini marshmallows before the dough sets.

Cover the dough and let it rest for at least 35 minutes to allow the chocolate to set.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Roll the dough into walnut-sized balls or drop by rounded tablespoonfuls onto the prepared baking sheets, leaving 2 inches of space between each cookie.

Bake for 8 to 10 minutes, or until the edges are set and the centers are still slightly soft.

Allow the cookies to cool on the baking sheets for 10 minutes before transferring them to wire racks to cool completely.