Using the bain-marie method, melt the butter in in a mixing bowl. Cut the Ragusa into small pieces, pre-heat the hot air oven to 180°C. Line a baking tray with waxed paper.
Place a piece of plastic wrap the same size as the strudel dough on a table. Spread the dough out on the plastic wrap and use a pastry brush to baste liberally with 2/3 of the melted butter.
Spread the ground hazelnuts, pieces of Ragusa and the whole hazelnuts over the dough and then roll up the dough while gripping the two edges and folding them under the strudel. Place the Ragusa strudel on the waxed paper, baste it with the remaining melted butter and bake for 30 minutes at 180°C in the hot air oven. Leave it to stand for a few minutes, then sprinkle with caster sugar, slice and serve hot or cold.