• Preheat oven to 350F / 177C / gas mark 4. Prepare 2 9-inch cake pans by spraying with cooking spray (I prefer Baker's Joy). Lining with parchment paper is optional.
• In a stand mixer (or in a large bowl with an electric hand-held mixer), beat butter, oil, and sugar together until creamy.
• Add eggs and egg yolk, one at a time, until combined after each addition. Add sour cream and vanilla and beat until just combined.
• Add flour and baking powder and stir by hand until just combined. Avoid over-mixing or the cake will turn out dense and tough. If the batter is too thick, try adding milk a tablespoon at a time, stirring gently.
• Divide batter evenly between cake pans, smoothing out the top. Bake for 25-30 minutes, or until cakes are golden and the top springs back when lightly pressed, or a skewer or toothpick comes out clean. (May have moist crumbs but should not have wet batter!)
• Allow to cool in pans for 10 minutes, then turn out onto wire racks to finish cooling completely.

Biscoff Cookie Butter Buttercream

• In a stand mixer (or large bowl if using electric hand mixer), beat butter for a few minutes to loosen.
• Add powdered sugar and mix slowly at a low speed until the sugar is incorporated. Increase speed and beat 3-4 minutes or until creamy.
• Add cookie butter and beat until thoroughly combined and creamy.
• If the buttercream is too thick, slowly add cream or milk, a little at a time, until it reaches a creamy, spreadable consistency. Beat for up to 5 minutes until it is perfectly creamy and smooth.
• Using a piping tip, pipe large dollops of buttercream onto the first layer of cake once it has completely cooled. You may also spread with a spatula, but make sure the buttercream goes to the edges so it is visible.
• Top with the second cake layer and pipe large dollops of buttercream on top and in the center. Top with Biscoff Cookies. Using a spoon, drizzle melted cookie butter over the top. Sprinkle with gold cake sprinkles or crumbled cookies as desired.