Make the pastry by creaming the butter and castor sugar together in a bowl. Add the egg and vanilla paste and mix in. Sift in the flour, baking powder and salt and mix to make a soft dough. Wrap in plastic wrap and refrigerate for an hour.
Preheat your oven to 180°C and grease a 24cm pie dish. Combine the blueberries, castor sugar and lemon juice in a bowl.
Peel, core and halve the apples, then cut into thin slices. Add these to the blueberry mix and stir.
Take the dough out of the fridge and keep approximately 80g aside. Dust your work surface with flour and roll the rest of the dough into a rough 26cm circle. Lift the circle and gently push it into your greased dish so that the dough goes up the sides. Keep any excess dough / trimmings and add it to the 80g that you set aside earlier.
Spread the dulce de leche over the base of the tart. Add the apple filling in an even layer. Grate the remaining dough over the apple filling (use a chunky grate).
Pop the tart in the oven for 35 - 40 minutes until the pastry is golden. Allow to cool slightly, then use a small piping bag or spoon to drizzle dulce de leche over the top. (If serving cold you can wait until the tart is completely cold before drizzling.)
Serving suggestion: the tart can be served warm or cold with cream, ice-cream or custard, (or on its own). Dust with icing sugar before serving.