Cut butter into pieces. Mix butter, flour, egg and icing sugar first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Wrap the dough in foil and chill for about 20 minutes. Roll out the dough on a floured work surface until round (approx. 28 cm Ø) and line a tart tin (approx. 24 cm Ø) with it. Prick the dough several times with a fork.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-30 minutes. Remove the tart from the oven and let it cool down. Soak the gelatine in cold water. Stir mascarpone, sugar and vanillin sugar until smooth. Stir in sour cream. Dissolve squeezed out gelatine in a small pot. Stir in 3 tablespoons of cream, then stir into the remaining cream. Halve the cream and mix one half with liqueur. Put the white cream and mint cream alternately into the tart and marble with a fork. Chill the tarte for at least 3 hours.
Stir in 3 tablespoons of cream, then stir into the remaining cream. Halve the cream and mix one half with liqueur. Put the white cream and mint cream alternately into the tart and marble with a fork. Chill the tarte for at least 3 hours. Peel and clean the melon and cut or slice into very thin slices/strips. Twist several melon strips into roses. Before serving, decorate the tarte with melon roses and mint leaves
4 1/2 hours waiting time