Melt butter over low heat and pour into a large mixing bowl (you could also use room temperature butter).
Add cocoa powder and powdered sugar to bowl.
Whisk until smooth.
Add milk (don't worry if the milk is cold and hardens the butter a bit.)
Add petit beurre to mixing bowl.
Mix with hands until combined.
Don't over-mix. You want to keep large pieces of biscuit along with the crushed pieces.
Line a bread pan with cling film and pack with the petit beurre mixture.
Smooth the top with a spatula.
Fold the cling film over the top.
Transfer to the freezer for 1 hour so the cake can set.
Slice and serve.
Can be stored in the fridge or freezer.
No-bake mosaic cake can be made with almost any type of biscuit or cookie. If you like a sweeter dessert, use Oreos, Vanilla Wafers, or any cookie you like. For a less-sweet version, like Turks make, use petit beurre or tea biscuits.