Preheat your oven to 375°F (190°C). In a food processor, crush Biscoff cookies into fine crumbs. Combine crumbs with melted unsalted butter and granulated sugar in a mixing bowl until well mixed. Press into the bottom of three greased 8-inch round cake pans. Bake for 5-7 minutes until lightly set and fragrant.

Lower the oven temperature to 325°F (163°C). In a medium bowl, whisk together flour, baking powder, pumpkin pie spice, ground cinnamon, ground ginger, nutmeg, and salt. In a stand mixer, cream together granulated sugar, light brown sugar, and eggs until light and fluffy. Gradually add vegetable oil, vanilla extract, and pumpkin purée, mixing until combined. Slowly incorporate dry mixture until just blended.

Divide the pumpkin cake batter evenly among the cooled Biscoff crusts in the prepared pans. Smooth tops with a spatula. Bake for 30-35 minutes, or until a toothpick comes out clean. Allow cakes to cool in pans for about 10 minutes before transferring to wire racks.

In a large mixing bowl, beat cookie butter spread and room temperature unsalted butter until well combined. Gradually add sifted powdered sugar, mixing on low speed until the filling is smooth and creamy. Once cakes are cooled, spread filling between the layers.

In a microwave-safe bowl, melt heavy whipping cream over white chocolate chips in 20-second intervals, stirring until smooth. In stand mixer, beat softened butter until fluffy, then incorporate sifted powdered sugar, melted chocolate mixture, vanilla extract, and a pinch of salt until light and airy.

Ensure each cooled cake layer is level. Place one pumpkin layer on a serving plate and spread cookie butter filling on top. Repeat with remaining layers and frost with white chocolate buttercream. Apply a crumb coat, then refrigerate for 10-15 minutes to set the frosting.

After chilling, add a final layer of frosting, smoothing it out with a cake scraper. Decorate top and sides with crushed Biscoff cookie crumbs. Let sit at room temperature for at least 30 minutes before serving.