Add butter and sugar into a bowl and mix them well using a spatula. Then add in egg yolk, vanilla extract and lemon zest. Mix well to combine.
Finally add in flour and mix just to combine. Don't over mix. Place the dough on a cling film and wrap it. Then place it in the fridge for 1 hour. Meanwhile let's make the filling.
Use a thick base pot and place it on medium heat. Add in the 3 cheeses together with milk and butter. Cook until the cheeses are melted.
Then add in icing sugar, corn starch and mix until smooth. Finally add in egg, vanilla extract and lemon juice. Continue cooking until the mixture thickens. Sift into a bowl to discard any lumps. Cover the bowl with the cling film touching the top surface of the filling and set aside to cool completely.
Remove the dough from the fridge after 1 hour. Take some of the dough and press it evenly inside a 2.5 inch (6.5 cm) tart mould. Cut off the excess dough with a knife and prick the base with a fork. Repeat until the rest of the dough is finished.Tip: Place the unbaked shell in a freezer for 10 minutes to firm up a bit.
Bake in a preheated oven on the lowest rack at 340ºF (170ºC) for 15 minutes. Cool completely and then remove the shells from the moulds. Tap the bottom a few times to help releasing it.
Fill the filling in a piping bag and pipe in inside the shells. Then place the unbaked tarts in the freezer for 30 minutes for the filling to firm up. Then brush the top of the filling with some egg yolk wash.
Grill for a few minutes in the oven until the top is slightly charred. Enjoy!