Place the salted butter in your mug (350ml/12oz capacity or more), then cover mug with a sheet of paper towel and place in the microwave to melt for 30 seconds.
40 g salted butter
Then mix in milk, light brown sugar and vanilla extract to the melted butter in the mug.
2 tablespoon milk, 30 g light brown sugar, ½ teaspoon vanilla extract
Add egg yolk and mix well.
1 egg yolk
Add self raising flour and mix until just combined, smoothing out any lumps of flour.
45 g self raising (self rising) flour
Add in chocolate chips a third at a time, gently pushing/poking chips into the batter each time, and scattering the final third over the surface without mixing.
1½ tablespoon chocolate chips
Place immediately into microwave and cook for 1 - 1½ minutes (checking at 1 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.