In a double boiler, melt the butter and chocolate, stirring frequently. Once melted, set aside to cool.
In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt.
In a standing mixer, beat the sugar, eggs and vanilla on low speed until combined. Add the cooled chocolate and mix until combined.
Gradually add in the flour ingredients and mix until combined.
Scoop the dough (will be brownie like texture) into a bowl and cover with foil and place into the fridge for 4 hours.
Preheat oven to 325 degrees and line a cookie sheet with parchment paper.
Using a cookie scoop, add some batter into your hands and roll into a ball.
Place cookies about 2 inches apart and bake the cookies for about 10 minutes.
While cookies are baking, cut the marshmallows in half.
Once cookies are baked, place a marshmallow into the center of the cookies and lightly press down. Place cookies back into the oven for another 2-3 minutes.
Place a wire rack over another cookie sheet. Once cookies are done baking let cool for a few minutes before transferring to a wire rack while you make the icing.