In a small bowl, whisk together flour, cocoa powder, sugar, and baking powder until thoroughly, completely combined with no streaks of flour or cocoa powder remaining. Blend in milk, butter, and vanilla until batter is smooth. Pour batter into a 14-ounce (or larger) microwave-safe mug with straight sides.
Combine peanut butter and chocolate chips and dollop into the center of the mug, gently pressing down until even with the top of the batter. Microwave on high for 1 minute. Allow to cool for a couple of minutes before serving.
A tall, cylindrical mug with straight (not angled) sides will ensure that the cake bakes evenly. I found a 14-ounce mug to be the perfect size to allow the cake to rise up while baking without overflowing. (I filled my mug to the top with water and then transferred the water to a measuring cup to determine its capacity.) If you use a smaller mug, you may want to bake the cake on top of a plate or paper towel to catch any possible overflow.
The strength of individual microwaves can vary. The first time you make this recipe, be prepared to experiment a bit to make sure that 1 minute is the perfect length of baking time. The nice thing about this cake is that even if it is undercooked, you don't have to worry about raw eggs!
The mini chocolate chips are optional. So is the peanut butter, for that matter. But I don't recommend skipping either. ;)
As an alternative to serving straight out of the mug, you may instead spray the mug with non-stick cooking spray before adding the batter, and then carefully overturn the cake onto a plate once baked.