Preheat oven to 350°F (175°C). Grease and line two 8-inch round cake pans or one 9x13-inch pan.
Mix the batter: In a bowl, combine cake mix, milk, melted butter, eggs, and melted Biscoff spread. Mix until smooth and creamy.
Bake: Pour the batter evenly into the prepared pans. Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean.
Cool completely before frosting to avoid melting the icing.
Make the frosting: Beat together butter and Biscoff spread until fluffy. Gradually add powdered sugar and milk until smooth and spreadable.
Assemble: Frost the cooled cake. Sprinkle crushed Biscoff cookies on top for decoration and texture.