Mist a large mug with cooking spray and set aside.
Combine the flour, sugar, and cocoa powder into the mug and whisk to combine. Add in the milk, oil, and vanilla. Stir and scrape the sides of the cup until the batter is mixed thoroughly. Stir in the chocolate chips.
Plop a spoonful of peanut butter directly on top of the chocolate cake batter. No need to mix.
Microwave for 1 minute. Remove and check with a toothpick. You may even be able to tell from looking at the center of the cake. Microwave up to 45 additional seconds, or until just done.
Serve immediately as is or with a drizzle of peanut butter or chocolate frosting.