On low, slowly stream in the melted chocolate mixture. Beat the mixture into the eggs and sugar until well combined. Mix in the powdered hot chocolate mix, espresso powder, baking powder, salt and flour.
Dip a large spoon into the glass or bowl of hot water. Scoop out a heaping spoonful of marshmallow creme. Drop the creme into the brownie batter/cookie dough. Gently swirl a little to create a slightly marbled look.