Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and Biscoff cookie butter until combined.
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Start and end with the flour mixture, mixing until just combined.
Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring to wire racks to cool completely.
For the frosting, beat the softened butter in a large bowl until creamy. Gradually add the powdered sugar, mixing on low speed until combined.
Add the Biscoff cookie butter, heavy cream, and vanilla extract, then beat on high speed until fluffy and smooth.
Once the cakes are completely cool, place one layer on a serving plate and spread a layer of frosting on top. Place the second layer on top and frost the top and sides of the cake.