Start by putting the flour, sugar, cocoa powder, and baking powder into a mug that’s safe for the microwave. Give everything a good stir so the dry ingredients are well blended.
Pour in the milk, the melted butter, and the vanilla. Mix until you have a smooth, even batter with no dry patches.
Spoon the peanut butter into the middle of the batter. Leave it as a little dollop doesn’t stir it in so it melts into a soft, gooey center as it cooks.
Scatter the mini chocolate chips over the top. Microwave the mug for roughly a minute to a minute and a half, just until the cake puffs up and the surface looks set.
Let the mug sit for a minute so it cools slightly the center will be very hot. Eat it warm, and add extra chocolate chips or a bit more peanut butter if you want.