Turn oven down to 190C/375F/Gas 5.
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 cooked pastry case. Bake for about 15 - 20 minutes, or until just set and still a bit wobbly in the centre. Remove from the oven and leave to cool in the tin before slicing and serving.