In a double boiler, over simmering water, melt the butter and 12-ounces of semi-sweet chocolate chips. Stir constantly until melted. Remove from heat, cool for 5 minutes.
In a large bowl, beat the brown sugar, eggs and vanilla, with an electric hand-mixer, on medium speed just until blended. Beat in the cooled chocolate mixture just until combined.
Add the cocoa powder, flour, baking powder and salt. Beat on low speed just until combined. Cover the bowl with plastic wrap and refrigerate for 2 hours.
Preheat oven to 325 degrees. Line two cookie sheets with parchment paper.
Scoop cookie dough by the tablespoon, roll into 1-inch balls, and place onto the prepared cookie sheets. Fit 12 cookies per sheet, making sure to space them 2-inches apart.
Bake cookies for 12 minutes. Carefully, remove cookies from the oven. Quickly dip cut-side of each marshmallow half into mini chocolate chips and place in the center of each cookie. Place cookies back in the oven and bake for another 4 minutes.
Remove pan from the oven, and allow cookies to cool at least 5 minutes before transferring to a cooling rack. Dust with cocoa powder if desired.