Add both the semi-sweet and white chocolate to a microwave safe bowl and add in the butter cubes. Place into the microwave and heat on high for 30 seconds. Take out, stir and repeat until the chocolate is smooth and creamy. Set aside to cool.
In a mixing bowl, sift flour, cocoa powder, salt and baking powder. Whisk to combine.
Fit your electric mixer with the paddle attachment. Add in the bowl the brown sugar, eggs, peppermint and vanilla. Then mix in the chocolate mix from step 1.
Mix together on low speed slowly adding the flour mixture.
Cover bowl and chill 1 hour.
Create balls of dough, about a tablespoon each and bake 9 minutes.
Remove cookies from oven and add 1 marshmallow half to each cookie. Bake again for another 3 minutes.
Remove from oven and let cool.