Make the dough: Mix flours, sugar, baking powder, and salt. Rub in cold butter until crumbly. Add eggs and vanilla and gently bring the dough together. Chill for 30 minutes.
Prep the apricots: Halve apricots, sprinkle with brown sugar, and bake at 350°F for 20 minutes until soft and lightly caramelized. Let cool.
Make the filling: Whisk mascarpone, sugars, cornstarch, egg, and vanilla until smooth and creamy.
Assemble the tart: Roll half the dough into a buttered 9-inch pan. Add apricots, spread mascarpone filling, then layer more apricots on top. Cover with remaining dough and seal the edges.
Chill & bake: Chill for 1 hour, then bake at 350°F for 45 minutes, until golden and set.
