In a standard 12-ounce microwave-safe mug, whisk together the protein powder, oat flour, and baking powder until the dry mixture is homogenous.
Integrate the Greek yogurt and almond milk into the dry mixture, stirring until a smooth batter with no visible dry spots remains.
Spoon the Biscoff cookie butter into the vertical center of the batter, pressing it down so it is flush with the surface to create a molten core.
Microwave the mug on high for 50 to 70 seconds. The timing is critical; stop when the surface is set but the cake still has a slight wobble in the center.
Allow the mug cake to rest undisturbed for 60 seconds to allow the internal structure to stabilize through carryover heat.
Garnish the top with the crushed Biscoff cookie and serve immediately.