Using the whisk on a hand mixer, beat the butter, icing sugar, vanilla sugar, salt and egg white in a bowl until the mixture becomes lighter in colour. Add the flour to the butter mixture, mix quickly to create a dough, gently flatten, cover and chill for approx. 2 hrs.
Divide the dough in half, roll out into rectangles (each approx. 2 mm thick) between two cut-open plastic bags, freeze for approx. 30 mins. On one half of the dough, spread approx. 1 tsp of jam at regular intervals. Brush the dough with the egg white. Place the other half of the dough on top, press down firmly, pressing out any trapped air. Cut out squares of approx. 5 cm using a pastry wheel, making sure that the jam is in the middle. Transfer the cookies to a baking tray lined with baking paper. Chill the ravioli cookies for approx. 15 mins.
Approx. 15 mins. in the centre of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, slide onto a cooling rack and leave to cool completely. Dip the cookies into the cake icing, leave to dry on a cooling rack.