Preheat oven to 350 degrees F. Line the bottom of two 8-inch cake pans with parchment paper and spray with nonstick spray. Set aside.

Cream butter and sugar: In a large mixing bowl with electric beaters or bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugar. Cream together for a few minutes until light and fluffy.

Add vanilla and then eggs, mixing in one at a time until combined. Add sour cream and 1/2 cup cookie butter and mix until well combined.

Dry ingredients: In a bowl, combine the flour, baking powder, and salt.

Finish batter: With the mixer on low speed, alternately add the dry mixture and the milk, starting and ending with the dry mixture.

Bake: Pour the batter into prepared cake pans and bake for about 22-27 minutes, or until a toothpick inserted in the center of the cake comes out clean (or with a few moist crumbs).

Cool cake in the pan for a few minutes, then invert cakes onto a wire cooling rack to cool completely.

Frosting: In a stand mixer fitted with the paddle attachment, combine the butter, 2 1/2 cups powdered sugar, and the reserved 1/2 cup cookie butter and mix until smooth. Add a splash of milk and additional powdered sugar as needed, mixing for several minutes until you reach a fluffy but thick and spreadable frosting consistency.

Assemble: Melt the remaining cookie butter in the microwave for a few seconds until runny (make sure the aluminum seal from the container is off). Add one cake layer to a serving plate or 8’’ cake board. Add a thin layer of buttercream frosting, and smooth most of the melted cookie butter on top (I like to reserve a spoonful to drizzle on the cake at the end), leaving a 1’’ border around the cake.

Top with second cake layer. Pipe a line of frosting into the crack between the two layers of cake. Frost cake completely with a very thin layer of frosting—the crumb coat. Stick in the fridge for 20 minutes or freezer for 10 minutes. Then, add another layer of frosting over the crumb coat.

Decorate: Pipe large decorative frosting swirls around the outer top edge of the cake. Drizzle a spoonful of reserved melted cookie butter in lines across the top of the cake. Sprinkle crushed Lotus Biscoff cookie crumbs on top.