For the crumbles melt butter in a pot. Mix flour and sugar in a bowl, add hot butter and knead everything into crumbles. Mix plum jam and cinnamon. Cut open the freezer bag.
Roll out the short pastry between the freezer bags to a 3-4 mm thin sheet. Peel off the upper freezer bag and lift it with the help of the lower one onto a greased baking tray sprinkled with flour. Spread with plum puree and spread the crumbles on top.
Bake in the pre-heated oven, mid-height (electric oven: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes. While still hot, cut into squares of approx. 4 x 4 cm and leave to cool on the tray.
Layer in layers with greaseproof paper in tins and keep cool. Dust with icing sugar before serving.