Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper. Stir flour, baking powder and salt in small bowl to blend. Set aside.
Stir 1 cup chocolate chips and butter in medium bowl set over saucepan of barely simmering water until melted and smooth. Transfer chocolate mixture to large bowl. Set aside.
Whisk sugar, egg, coffee mixture and vanilla in medium bowl until blended. Stir egg mixture into reserved chocolate mixture. Mix in flour mixture just until blended. Add nuts and remaining 1 cup chocolate chips.
For each cookie, drop 1 rounded tablespoon dough onto prepared baking sheet, spacing 3 inch apart. Gently press 5 marshmallows into center of each. Gently press 1 rounded tablespoon of dough atop each to cover marshmallows and form 2-inch-high mounds. Bake cookies until top is dull and feels crusty when touched gently, rotating baking sheets front to back and top to bottom halfway through baking to ensure even browning, about 14 minutes.