Melt the butter and chocolate in the microwave. In a stand mixer fitted with the whisk attachment, whisk the sugar and eggs together on medium speed until light, fluffy and pale yellow.
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.
Using a large cookie scoop, scoop out 3-4 large cookies. Drop the cookie dough onto the prepared baking sheets. Leave 2-3" of space in between each cookie.