Sift the flour into a bowl. Add in granulated sugar and give it a mix. Add in soften butter, vanilla extract and beaten egg. Using a spatula, mix to combine until the mixture becomes like a wet sand. Add milk and using your hand, mix and gently form into a dough. Do not overwork the dough to prevent hard, dense pastry.
Wrap with cling film and leave to chill in the fridge for 30 minutes. Meanwhile we can prepare our filling.
In a bowl, add in all the filling ingredients. Mix just to combine. Do not over mix. Set aside in the fridge.
Place ⅔ of chilled dough on a cling film. This will be the base. Press a bit and cover with another piece of cling film. Gently roll into an 11 inch (28 cm) diameter disk. We will be using a 9 inch (23 cm) spring form with 3 inch (8 cm) depth. Use the pan as a guidance. Slide the rolled dough onto a baking tray and chill in the fridge for 10 minutes. This is to make it a bit firmer and more manageable later.
After 10 minutes, remove the dough from the fridge and place it on a working surface. Remove the top cling film. Sprinkle some flour on the dough surface and coat it well. The flour helps the dough not to stick and make it easier to adjust to the pan.
Place a 9 inch (23 cm) spring form pan (buttered and floured before hand) up side down on the down. Slide one hand down under the dough and swiftly flip them both. Remove the remaining cling film and adjust the dough to the pan. The dough is very forgiving. Patch here and there as needed and discard the excess dough aside.
Pipe 1 cm height of bakers dulce de leche, covering the base of the dough. Spoon all of the previously prepared filling on the piped dulce de leche. Spread evenly.
Roll out the remaining ⅓ dough and place it on the filling. Adjust accordingly so that it covers all the filling surface. Lightly press both of the surrounding base and top dough together. Cut excess dough on the side, leaving around 2 cm of height. Pinch the seams together. Use the excess dough to make some decorations, if you want. Brush the pastry with egg wash.
Bake at 175ºC (350ºF) in a preheated oven for 45 minutes, or until golden brown and the sides detached from the pan. That indicates that it's cooked. Very important to leave it to cool for 15 minutes and only then remove it from the pan. Then leave the pie to cool completely before leaving it in the fridge overnight before serving. Enjoy!