Clean and wash the rhubarb and cut it into pieces of the same size and about 3 cm long. Cream butter and sugar. Stir in eggs and egg yolks one after the other. Mix flour, baking powder and sauce powder. Alternately stir in milk and flour mixture.
Spread the dough on a greased baking tray (32 x 38 cm). Spread 2 rhubarb pieces alternately lengthwise and crosswise in straight lines on the dough. Bake in the preheated oven (electric oven: 175 °C/ convection oven: 150 °C/ gas: level 2) for 25-30 minutes. Let cool, cut into pieces and serve sprinkled with icing sugar and coloured sugar