To make the cookie butter cake:

• Preheat the oven to 350°F. Spray three 8″ cake pans with baking spray and line the bottom of each with parchment paper.
• In the bowl of a stand mixer fitted with the paddle attachment, add the butter and cookie butter and beat on medium speed until combined and creamy. Add the vegetable oil and beat on low speed until homogenous with the butter.
• Next, add the sugar and brown sugar to the mixing bowl, and beat on medium speed until combined and light in color, about 2 minutes. Scrape down the sides of the bowl. One at a time, add the eggs, mixing on medium speed for 20 seconds after each addition. Mix in the vanilla.
• In a separate bowl, combine the sifted flour, baking soda, baking powder, kosher salt, ground cinnamon and nutmeg. In another separate bowl or large liquid measuring cup, whisk the sour cream and buttermilk together until smooth and combined.
• Beginning and ending with the flour mixture, alternate adding the flour mixture and the buttermilk mixture to the batter, mixing on low speed until just incorporated after each addition. Be careful not to overmix. Divide the batter evenly between the prepared cake pans (NOTE: I like to do this by weight. See my notes above.).
• Bake for 28 to 32 minutes, until the edges of the cake are pulling away from the sides of the pan and a toothpick inserted into the cake comes out clean. Allow the cakes to cool in their pans for 10 minutes before turning out onto a wire rack to cool completely.

To make the Biscoff Swiss meringue buttercream:

• Combine the egg whites, sugar and kosher salt in a heatproof bowl set over a pot filled with 2″ of simmering water, being sure that the water does not touch the bowl with your ingredients. Whisk constantly until the sugar has dissolved and an instant read thermometer registers 160°F, about 7 to 9 minutes.
• Carefully transfer the egg white mixture to the bowl of a stand mixer fitted with the whisk attachment. Whisk on high speed for 7 minutes; the mixture should look light and fluffy.
• Reduce the mixer to medium-low speed. Add the butter, 2 tablespoons at a time, making sure each portion of butter is incorporated before adding the next. Once all the butter has been incorporated, increase the mixer to high speed and whip for 8 minutes, until the meringue is glossy and fluffy. Add the vanilla and cookie butter and mix on high until fully combined. Use immediately on the cooled cakes.

Assembly:

• Place a small dollop of buttercream on the center of a cake turntable or cake stand. Place the bottom cake layer over the dollop of buttercream and gently press to secure to the surface.
• Next, spoon about 1/2 to 3/4 cup of buttercream onto the bottom layer and smooth into an even layer using an offset spatula. Carefully place the next cake layer on top and very gently press to adhere the two layers together. Repeat this process with the third layer.
• Apply a crumb coat. To do this, apply a thin layer of buttercream to the entire cake, smoothing with an offset spatula or icing smoother. Place the cake in the fridge for 20 minutes. Allowing the initial layer of buttercream to set in the fridge makes applying the final layer of buttercream easier. (NOTE: check your bowl of buttercream towards the end of the chill time. If the buttercream seems soupy, pop it in the fridge to chill for 2 to 3 minutes to firm up.)
• Remove the cake from the fridge. Apply a layer of buttercream over the surface of the cake and finish as desired. To recreate the finish I chose for this cake, use a combination of an icing smoother and offset spatula to create a smoother look. Sprinkle the top outer edge of the cake with crushed Biscoff cookie crumbs, then finish by piping swirls around the perimeter of the cake using a large open star tip (I used an Ateco #828).