Place salted butter in microwave-safe mug, then cover the top with a sheet of paper towel and microwave to melt.
35 g salted butter
Add sugar, milk and vanilla extract then mix to combine.
30 g white (granulated) sugar, 2 tablespoon milk, ½ teaspoon vanilla extract
Add self raising (self rising) flour to wet ingredients and mix until just combined.
60 g self raising (self rising) flour
Spoon out some of the batter mixture, then sprinkle about half of the cinnamon sugar mixture over the remaining batter in the mug.
Then plop that spoonful of batter back in and sprinkle the remaining cinnamon sugar on top - don't mix!
Place mug (uncovered) immediately into the 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 second bursts.
Allow to cool slightly before eating.