Melt butter in a small bowl in the microwave and let cool slightly.
Mix flour, sugar, baking powder, and salt in a microwave-safe mug.
Stir in milk, vanilla, and the butter.
Stir in Biscoff cookie spread.
Break Biscoff cookie into small pieces, and stir in.
Microwave for 75 seconds on high.
Top with whipped cream and more biscoff cookie crumbs, if desired.
Notes
We recommend choosing a mug with straight sides—that makes it easier to mix the cake batter, and also let the cake rise as it bakes. Make sure to mix batter well in mug to make sure there is not a big chunk of flour on the bottom. Neaten the edges of the mug with a spatula or wipe the rim of your mug with a damp paper towel after adding all of the ingredients. This will ensure your cookie butter mug cake cooks and rises evenly. Makes sure to let your cake cool slightly before eating, because the middle can be extremely hot when it comes out of the microwave.Note that microwaves are all different—you might need to microwave for more or less time depending on your appliance. You want the surface of the cake to be fully cooked, with possibly a small area that still looks a little wet. Cake will continue to cook as it sits. And of course, if you like your cake gooey, you can cook it a little less.