Prep the Pans: Line three 6-inch cake pans with parchment paper and preheat oven to 350°F (180°C).

Mix Dry Ingredients: In a stand mixer, combine cake flour, sugar, baking powder, and salt. Add unsalted butter and cookie butter, mixing on low until resembling coarse crumbs.

Combine Wet Ingredients: Add sour cream and oil, mix into a thick paste. Gradually add milk, eggs, and vanilla extract, mixing until smooth.

Layer Biscoff: Divide half the batter in prepared pans, drizzle melted cookie butter over, top with remaining batter and bake for 30-35 minutes.

Cool: Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.

Make Buttercream: Beat together softened unsalted butter and cookie butter until creamy, add powdered sugar, heavy cream, and vanilla extract.

Assemble the Cake: Slice each layer in half, layer with buttercream and cookie crumbs until stacked.

Decorate: Crumb coat the cake, chill, apply a thicker layer of frosting, drizzle with melted cookie butter and sprinkle with cookie crumbs.