Preheat your oven to 350°F (175°C). Grease your cake pans with cooking spray or butter.
In a large mixing bowl, beat together the unsalted butter and granulated sugar until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until combined.
Mix in the sour cream and half of the cookie butter until smooth and creamy.
In a separate bowl, whisk together the all-purpose flour, baking powder, and kosher salt.
Gradually add the dry ingredients to the wet mixture, alternating with the whole milk. Start and end with the flour mixture, mixing until just combined.
Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
In a medium bowl, beat the slightly cold unsalted butter and remaining cookie butter until smooth. Gradually add in the powdered sugar and milk, mixing until you reach your desired consistency.
Once the cakes are completely cool, place one layer on a serving plate. Spread a generous layer of frosting on top, then place the second cake layer on top and frost the top and sides of the cake.
Drizzle additional cookie butter over the frosted cake and sprinkle crushed Biscoff cookies on top for decoration.