Prep your pan and oven: Heat oven to 350°F (175°C). Grease and line a 9x13-inch pan with parchment, leaving overhang for easy lifting. For a layer cake, use two 8-inch round pans, greased and lined.

Mix dry ingredients: In a large bowl, whisk 2 1/2 cups flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, and 1 teaspoon cinnamon.

Combine wet ingredients: In another bowl, whisk 1/2 cup melted butter, 1/4 cup neutral oil, 3/4 cup granulated sugar, and 3/4 cup light brown sugar until glossy. Whisk in 3 large eggs, one at a time, then 1 tablespoon vanilla.

Add cookie butter and dairy: Warm 2/3 cup cookie butter in the microwave for 10–15 seconds until loosened. Whisk it into the wet mixture along with 3/4 cup sour cream and 1/2 cup milk until smooth.

Combine batter: Add the wet mixture to the dry ingredients and stir gently until just combined. Do not overmix. Fold in 1 cup roughly crushed Biscoff cookies for pockets of crunch.

Bake: Pour batter into the prepared pan and smooth the top. Bake 28–34 minutes for a 9x13, or 24–28 minutes for 8-inch layers, until a toothpick comes out with a few moist crumbs.

Cool completely: Let the cake cool in the pan for 10 minutes, then transfer to a rack to cool fully before frosting.

Make the buttercream: Beat 1 cup (2 sticks) softened butter on medium-high for 2 minutes until fluffy. Add 3/4 cup cookie butter and beat until smooth. Gradually add 3–3 1/2 cups powdered sugar, mixing on low. Add 2–4 tablespoons heavy cream, 1 teaspoon vanilla, and a pinch of salt. Beat on high for 1–2 minutes until light and silky.

Frost and finish: Spread buttercream over the cooled cake. Top with extra crushed cookies and a warm cookie butter drizzle. Add a tiny pinch of flaky sea salt if you like contrast.

Slice and serve: Let the frosting set for 15–20 minutes for cleaner slices. Enjoy at room temperature for the best texture and flavor.