In a large mug or small bowl, whisk together the flour, sugar, baking powder, and cinnamon.
Pour in the milk, vanilla, and melted butter. Add in 2 tablespoon of cookie butter, mixing until everything is combined. Before baking, place 1 tablespoon of cookie butter on top of the batter in the center of the mug.
Microwave the mug cake for 1 minute and 30 seconds to start. If it doesn't seem almost cooked, put it back in for another 10-15 seconds. 1 minute 30 seconds was perfect for me, but every microwave is different. You don't want to overbake your mug cake either, so it's better to underbake at first.
Once your cake is done baking, allow it to sit for a minute or two to cool off, and then crumble a biscoff cookie on top.
Notes
Use a microwave-safe mug- this may seem like a no-brainer, but make sure your mug is microwave-safe! I would also be careful taking your mug out after baking the cake, as it is usually pretty hot. Over-size mugs are the way to go for mug cakes so your cake has room to rise when baking.
Spoon and level your flour- I always recommend spooning and leveling your flour when baking. Scooping the measuring cup right into the flour will actually compact the flour and give much more than the recipe calls for. To spoon and level, use a large spoon to gently scoop the flour into your measuring cup. Once it's full, use a flat edge (like a butter knife) to gently scrape across the top of the measuring cup.
Don't overmix- Just like any cake batter, if you overmix it, it will turn out dense and tough. I suggest just mixing until the streaks of flour are gone.