Pre-heat your oven to 350ºF.
Place the sliced cherry tomatoes onto a platter and season them with salt and pepper. Add the 2 tablespoons of chopped Basil and set the tomatoes aside.
For the dough, place the all-purpose flour and salt into a food processor bowl and blitz it to combine.
Add the butter and process it until the butter resembles fine bread crumbs.
Add the egg and water or milk and mix until a dough forms.
Knead the Empanada dough lightly, then divide into four equal portions.
Roll out each portion of Empanada dough until it is 1/2-inch in thickness, then cut 2 rounds out of it.
Add a slice of Mozzarella to the center of each empanada round, then add a tablespoon of tomato basil mixture to the empanada dough, leaving a 1/2-inch border around the center of the dough.
Dab the edges of the Empanada dough with water, fold one-half of the dough over the filling, press to seal it, then press the edges of the dough with a fork to seal it.
Arrange the Empanadas onto a parchment-lined cookie sheet, and if possible let them rest in the fridge for about 20-30 minutes for a better result.
When you're ready to bake your Empanadas, whisk the egg yolk in a bowl and brush the Empanadas.
Bake the Empanadas for 20-25 minutes until golden brown.
Serve and enjoy!