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.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined.
Fold in ½ cup of the Biscoff cookie butter and the crushed Biscoff cookies until evenly distributed.
Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.
Once cooled, place one cake layer on a serving plate. Spread the remaining ½ cup of Biscoff cookie butter on top. Place the second cake layer on top and frost the entire cake with your favorite frosting or serve as is.
For best results, chill the cake for at least 30 minutes before slicing to allow the filling to set.