Place the salted butter in your mug (350ml/12oz capacity or more), then cover the mug with a sheet of paper towel and place it in the microwave to melt for 30 seconds.
35 g salted butter
Add peanut butter and mix until melted and combined.
40 g peanut butter
Mix in white sugar, vanilla essence and milk.
30 g white sugar, ½ teaspoon vanilla essence, 2 tablespoon milk
Add egg yolk and mix well until fully combined.
1 egg yolk
Add self raising flour and mix until just combined, smoothing out any lumps of flour.
35 g self raising (self rising) flour
Add chocolate chips a third at a time, gently pushing/poking chips into the batter, and scattering the final third over the surface without mixing.
1 tablespoon chocolate chips
Place mug immediately into the microwave and cook for 1½ - 2 minutes (checking at 1½ minute mark) depending on your microwave.
The cake is done when it's puffed up, but will still feel spongey to the touch and a little tacky on top. If it needs more cooking, do so in short 10 - 15 second bursts.
Allow to cool slightly before eating.