The Polish recipe calls for budyn, a powdered custard which might be tricky to find outside of Poland, so I used thick pouring custard instead. You can bake the pastry form-free or if you happen to have a 30cm x 20cm cake tin, use that.
Tip: store the slice in the fridge and consume within 2 days. Bring to room temperature before serving. You may need to re-dust with some icing sugar.