Spray your mug or ramekin with nonstick spray then combine the dry ingredients. This includes the flour, cocoa powder, sugar, baking powder, and salt. Stir until everything is combined and there are no clumps of cocoa powder left.
Add the wet ingredients, oil and milk, to your mug. Stir everything together until smooth. Mix in the chocolate chips.
Lastly, add the tablespoon of peanut butter or Nutella to the center of the cake batter and gently push down.
Place the mug in the microwave and cook on high power for 45 - 60 seconds, or until the cake is cooked through. Be sure to watch the mug while it's cooking so that it doesn't overflow or burn.
Once cooked, you can enjoy the mug cake right away! It tastes delicious by itself or topped with a scoop of ice cream and some extra chocolate chips on top. Enjoy your homemade mug cake!