In a 10-12oz microwave-safe mug, mix together the Biscoff cookie butter, milk, and oil until it's well combined.
Add the sugar and give it a good stir.
Sift in the flour, baking powder, vanilla extract, and a pinch of salt. Gently stir until everything is combined, making sure to scrape the edges and bottom of the mug.
Clean up the edges of the mug, then microwave it on high for 60-90 seconds (I find 80 seconds perfect for this cake).
Serve it warm, and for an extra treat, spread some more Biscoff cookie butter on top!