Preheat oven to 160С degrees.
Spread 2 small baking pans (or 1 medium sized) with the butter.
Wash the fruits. Cut the plums into pieces, mix with the blackberries, add 1 tbsp sugar and mix together. Set aside.
Mix the flour, baking powder, sugar and salt in a medium bowl. Add cubed cold butter and rub with your fingertips until it crumbs. Mix the egg yolks and milk in a small bowl and pour into the flour. Mix together using spatula until you get a soft dough (don't mix too much). Divide into 6 pieces and place over the fruits lightly pressing with your hand.
Bake 40 minutes until the crust is golden brown.
Serve warm with a scoop of vanilla ice cream or a tbsp of double cream.