Line a 23cm tart tin with the pastry and place baking parchment or foil on top. Fill with ceramic baking beans or dried beans and bake for 15 minutes, then remove the parchment and baking beans and bake until lightly coloured, about 8 more minutes.
Turn oven down to 190 C / Gas 5.
For the filling, beat both milks and the dulce de leche in a large bowl, starting off slowly then increasing the speed, for around 10 minutes until the mixture is increased in volume, light and bubbly.
Pour mixture into the pastry case. Bake until just set and still a bit wobbly in the centre, 15 to 20 minutes. Remove from the oven and leave to cool in the tin before slicing and serving.