Preheat oven to 200℃ (180℃ fan forced). Line 2 baking trays with baking paper.
Using a sharp knife, slice each sheet of pastry into 5 long triangles the length of the pastry. Some wont be perfect triangles but this is ok.
Lay the triangles out on a clean work surface. Roughly spread each triangle with 1 teaspoon of the dip. Top with a few basil leaves and a strip of prosciutto. Roll up each triangle from the wide end to the short end. Place onto prepared trays leaving room between each one.
Brush croissants with egg yolk to cover and bake in the oven for 20-25 min. Serve warm or at room temperature.