Preheat the oven to 220°C and line two baking trays with baking paper. Roll out the dough and cut each sheet of pastry across the diagonals to give you four triangles. Dot cubes of butter on top of each triangle. Starting at the base of each triangle, loosely roll it upwards towards the tip. Place the finished croissants on the lined baking trays, curving them into crescent shapes. Brush with egg wash and bake for 15 minutes. Reduce the temperature to 190°C and bake for another 4-5 minutes, until they are evenly golden brown. Leave them on the tray to cook internally for another 10 minutes. Serve warm or at room temperature with preserves, cheeses and cold cuts.