Add the salted butter to a mug (see my notes for tips on size etc), cover with a sheet of paper towel then place in the microwave and heat for 30 seconds to melt.
30 g salted butter
Add the brown sugar to the melted butter and mix to melt the sugar slightly.
30 g light brown sugar
Add the Biscoff and stir until smooth, then stir in vanilla and milk.
40 g Lotus Biscoff cookie butter, ½ teaspoon vanilla, 2 tablespoons milk
Add egg yolk and stir until combined.
1 egg yolk
Add self-raising flour and cinnamon and mix until just smooth and combined (don't overmix).
35 g self-raising (self-rising) flour, a pinch of cinnamon
Microwave on high for 1½ - 2 minutes (check at 1½ minutes) 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 longer, do so in short 10 - 15 second bursts.
Serve immediately, decorated with a Biscoff cookie, some extra Biscoff (warmed) drizzled on top and sprinkled with Biscoff cookie crumbs (if using).
1 Biscoff cookie, Biscoff cookie crumbs