Line a 8 x 8 square pan with foil overlapping edges. Butter lightly.
In a small saucepan over very low heat combine sweetened condensed milk and chocolate chips. Stir together and heat until melted. Remove from heat and add vanilla.
Pour or spoon into square pan. Chill for at least 15 minutes.
Meanwhile, over low heat combine butter and cookie butter (biscoff spread) in a small sauce pan. Stir and heat until melted. Add sweetened condensed milk. Remove from heat. Stir in powdered sugar and vanilla.
Pour over slightly hardened dark chocolate layer. Spread evenly. Chill for another 2 hours.
Lift from pan. Cut into 1 inch pieces. Keep refrigerated.