Warm up the milk. Slowly melt 50 g sugar at medium heat and let it caramelize golden. Add milk and simmer gently for about 5 minutes. Chop the walnuts. Add 1 egg to the caramel milk, stir until smooth and stir at low heat until a slightly thick-creamy consistency is obtained.
Do not let it boil! Set aside. Beat the fat and 25 g sugar until fluffy. Stir in 1 egg. Mix flour, walnuts and baking powder and stir in together with the caramel cream. Chill the dough for about 30 minutes.
In the meantime, wash the plums, drain them, cut them crosswise and remove the stones. Stir the sour cream, 3 eggs and 125 g sugar until smooth. Put the dough into a greased and breadcrumbed tart tin (approx. 28 cm Ø) and press it down with floured hands.
Place the plums in the tart tin with the tips facing upwards. Pour over the sour cream and bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) on the lower rack for approx. 1 hour.
Remove the tart, let it cool down and take it out of the mould. Cut open and decorate with mint.