For the rhubarb and raspberry cake, peel the rhubarb, cut into thin slices, mix with the sugar and leave to stand.
For the base, cream the butter with the sugar and lemon zest. Beat in the eggs one at a time until well blended. Mix the flour with the baking powder and sift over the butter mixture, mixing well.
Line a 24 cm springform pan with baking paper and spread the batter on it. Top with rhubarb pieces and raspberries.
For the glaze, whisk the yolks with the sugar, vanilla sugar, crème fraiche and whipped cream and spread over the fruit.
Bake at 200 °C for about 30 minutes on the middle shelf.
For the meringue, beat the egg whites with the sugar until stiff. Spread over the cake with a piping bag and bake for another 2-3 minutes at top heat.