Roll out the dough to a thickness of 2-3 mm. and cut into 8 rectangles of about 10 x 12 cm. Lay on baking paper in a baking tin and brush with egg yolk. With a fork trace wavy lines in the dough. Bake for about 10 mins. in the middle of the oven, preheated to 200°C. Allow to cool.
Cut the leek into slices. Sauté in the butter with the curry powder and mix in the flour. Pour on the stock, bring to the boil and allow to simmer for 10 mins. Stir in the single sour cream and season.
With a sharp knife halve the puff pastries horizontally. Lay the lower halves on ovenproof plates and spread 2-3 tbs of the leek mixture on each one. Lay the Vacherin slices on top and melt under the oven grill.
Sprinkle paprika on the cheese and leek and half cover with the puff pastry lids. Serve, accompanied by the rest of the leek mixture.