Begin by preheating your oven to 350°F (175°C). Grease two 8-inch round cake pans and line the bottoms with parchment paper for easy removal. Lightly dust the sides with flour or use baking spray.

In a large bowl, cream together the softened butter and granulated sugar using an electric mixer until light and fluffy (about 3–4 minutes). Add eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In a separate bowl, sift together the flour, baking powder, and salt. Gradually add the dry mixture to the wet ingredients, mixing on low speed. Pour in the milk slowly and continue mixing until just combined. Fold in the crushed butter cookies using a spatula.

Divide the batter evenly between the prepared pans. Smooth the tops and bake for 35–40 minutes, or until a toothpick inserted into the center comes out clean. Remove from oven and let cool in pans for 10–15 minutes, then transfer to a wire rack to cool completely.

While the cakes cool, crush the Biscoff cookies in a food processor until fine crumbs form. In a large bowl, beat the butter until pale and creamy (about 2–3 minutes). Gradually add the powdered sugar, mixing on low at first, then increasing speed. Stir in vanilla, salt, Biscoff spread, and milk (or cream) and beat until smooth. Fold in the crushed cookies for texture.

If the cake layers are domed, use a serrated knife to level them. Place one layer on a serving plate or cake stand. Spread a generous amount of buttercream on top. Add the second layer and frost the top and sides of the cake with the remaining buttercream. Smooth it out with an offset spatula.

If desired, top with extra cookie crumbs, whole Biscoff cookies, or a drizzle of warmed cookie butter. Chill for 15–20 minutes before slicing for cleaner cuts. Serve at room temperature for the best flavor.