Mix the butter, caster sugar, vanilla sugar, lemon zest and lemon juice and stir to a soft, spreadable filling (remonce).
Unroll the puff pastry from its paper and spread the filling over the whole surface.Leave a couple of cm free along one of the long edges.
Grate the marzipan over the base and sprinkle with the flaked hazelnuts – save a few for the top.
Now fold or roll the puff pastry up from the long side, so it forms several layers. Finish with the seam facing down and press it lightly flat – without squashing it.If you like, see the video just above the recipe.
Cut the puff pastry log into triangles.
Brush the slices with beaten egg and sprinkle with the remaining flaked hazelnuts and a little pearl sugar, if using.
Bake in a preheated conventional oven (top and bottom heat) at 200°C for about 18-20 minutes.
Serve warm, then store in an airtight bag at room temperature. They stay soft and sticky for a couple of days.