Finely grate the marzipan. Separate the egg. Mix marzipan, egg white and sour cream with the whisk of the hand mixer until creamy. Wash the plums, drain well, cut in half and stone. Cut the plums into very fine slices.
Take the puff pastry out of the fridge and let it rest for about 10 minutes. Grease a tart mould (24 cm Ø) with lift-off base. Roll out a puff pastry, turn it over onto a floured work surface and remove the baking paper.
Cut off a strip approx. 2.5 x 25 cm wide and a strip approx. 8 x 25 cm wide. Place the remaining dough in the mould so that the dough is over the edge of the mould on three sides. Now place the approx. 8 cm wide strip in the mould so that the mould is lined with the dough and the dough is over the edge of the mould on all sides.
Press the dough seam well together and cut off the excess dough all around to about 1 cm above the edge. Prick the dough base several times with a fork and spread with about half of the cream.
Unroll the second dough, turn it onto a floured work surface and remove the baking paper. Cut the dough into approx. 15 strips (each approx. 2.5 x 25 cm). Spread these 15 strips and the remaining strips from the first pastry sheet thinly with the remaining marzipan cream.
Place a strip of sliced plums on top of each, so that the slices are 1-2 mm above the edge of the dough on one long side with the rounded skin side 1-2 mm above the edge. Leave an approx. 1.5 cm wide edge at the ends of each strip.
Carefully roll up the dough strips tightly and press the free end.
Arrange the plum roses (approx. 16 pieces) in the tart tin so that approx. 0.5 cm is left free between each rose. Place the tart in the preheated oven on the lower shelf (electric cooker: 225 °C/ convection oven: 200 °C/ gas: see
manufacturer) bake for about 40 minutes. Whisk the egg yolks. About 5 minutes before the end of the baking time, brush the tart with egg yolk and bake until golden brown. Let the finished tart cool down in the tin on a cake rack for about 15 minutes, then remove from the tin and let it cool down.