Melt the Biscoff spread in a large mug by microwaving it on high for 20 seconds.
Add in all the remaining ingredients into the mug. Mix to combine.
Microwave on high for 1 minute 30 seconds. If the cake hasn't been cooked through, cook at 10-second increments until set.
Allow to cool down slightly before adding toppings as you desire. Serve warm.
Notes
A wide mug is preferable as it would allow heat to distribute more evenly when cooking in the microwave.
Melt the Biscoff spread before mixing. Otherwise, it would be rather difficult to mix properly.
It is critical to not overcook a mug cake. Cooking times may differ from microwave to microwave. Therefore, test at 1 minute 30 seconds and then, if required, cook at further 10-second increments until set.