Combine ingredients for the filling and set aside until needed.
On a floured surface, lay the puff pastry and roll so it's extra thin, it should be 1.5mm/2mm in thickness.
Use the cutter to cut as many rounds as the pastry will allow. When re-rolling the pastry scraps, make sure to lay the pastry pieces on top of one another (rather than bunching into a ball) as this helps keep the pastry flaky.
Scoop a teaspoon of the fruit filling into the centre of each circle. Wet the perimeter of the circle with a damp finger, and pinch the edges of the pastry together to cover the filling. Flip the cake over so the smooth side is facing up, and using your hands and a rolling pin, press and roll out the cake until it's a larger circle and the fruit peeks through the pastry. Repeat for the rest of the pastry and filling.
Place the cakes on a baking tray and, using a sharp knife, cut three slits. Brush with egg white and sprinkle with Demerara sugar.