1In a microwave-safe mug, melt the butter by microwaving it for about 10-15 seconds.
Add the flour, sugar, and milk to the mug and mix well until smooth.
Crack the egg into the mug and mix thoroughly until the batter is well combined.
Crush the Biscoff cookies into small pieces and fold them into the batter.
Microwave the mug on high for 1-2 minutes, or until the cake has risen and is cooked through.
Let the mug cake cool for a minute before enjoying.