Preheat the oven to 370℉/180℃. Grease a 9 inch circular cake pan or skillet.
In a large bowl, using an electric/stand mixer, combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again.
Add the flour, baking powder and chocolate and fold until combined. The dough should be fairly solid at this stage.
Press half the dough into the tin/pan. Scatter over some marshmallows to mostly cover the cookie dough. Cover with remaining cookie dough - pressing together (don't worry if not all marshmallows are covered).
Bake 20-25 minutes, until golden on top. Remove and only let cool a few minutes before serving. Serve while still warm!
