Cream the butter and sugar, then mix in the egg yolk. Add flour, ground almonds, baking powder, and salt, and knead into a smooth dough. Chill.
Drain the mandarins well and reserve the juice. Mix about 2–3 tbsp of the juice with the cornstarch, bring the rest to a boil, stir in the starch mix, and cook briefly until thickened. Stir in the vanilla extract. Let cool.
Rub together all crumble ingredients with your fingertips until crumbly.
Roll out the dough and cut out circles (about 7 cm). Place directly into tartlet molds (just covering the base, not the sides).
Spoon 1–2 tsp of the mandarin filling onto each base and top with crumble.
Bake in a preheated oven at 180 °C (350 F) top/bottom heat for about 30 minutes, until edges are lightly golden.
Let cool and dust with powdered sugar.
