Grab a microwave-safe bowl and toss in the flour, sugar, cocoa powder, baking powder, and salt. Give it a good whisk to mix everything up.
Pour in the melted butter, crack in the eggs, and add the milk. Stir it up until you've got a smooth batter.
Pop it in the microwave on high for one minute (or up to 90 seconds depending on your microwave). Stick a toothpick in the center—it should come out clean. Be careful not to overcook; we want a moist cake, not a rubbery one!
Cut a small piece from the center of the cake and set it aside.
Spoon that marshmallow fluff goodness into the hole you made and put the cake piece back on top.
Serve it up right away and enjoy your quick, indulgent treat!