Preheat the oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
Add the flour, sugar, baking powder, salt, and cinnamon to a large bowl. Add the cold cubed butter and work it into the flour until coarse crumbs form.
Add the egg and vanilla. Mix gently until the mixture forms moist, uneven crumbs. Do not knead it into a smooth dough.
Reserve about one-third of the crumb mixture for the topping. Press the remaining mixture firmly into the bottom and slightly up the sides of the prepared pan.
Add the diced apples, brown sugar, cinnamon, lemon juice, and butter to a skillet. Cook over medium heat for about 5–7 minutes, until the apples begin to soften. Remove from the heat and let cool slightly.
In a separate bowl, beat the mascarpone and sugar just until smooth. Add the eggs, vanilla, cornstarch, and salt. Mix until creamy and combined.
Spread about half of the cooled apples over the prepared crust.
Pour the mascarpone mixture over the apples and gently smooth the top.
Scatter the remaining apples over the mascarpone layer.
Sprinkle the reserved crumble mixture generously over the top.
Bake for 50–60 minutes, until the crumble is golden brown and the mascarpone filling is mostly set with a slight softness in the center.
Cool completely in the pan, then refrigerate for at least 3 hours before removing from the springform pan.
Dust lightly with powdered sugar before slicing.