Make the pie crust:
In a bowl of a food processor, add the flour, sugar, and salt. Add the cubes of butter and pulse about 8-10 times until the flour is incorporated with the butter, and the butter is in pea sized pieces. It is totally cool if the butter pieces are larger than pea size.
In a separate bowl, beat the egg with the milk, and then add them all at once to your dry ingredients. Pulse a few times until the flour is moistened. This will take about 7-8 pulses and you'll be left with a very wet looking dough which will resemble sugar cookie dough. This is fine.
Turn the dough over a lightly floured surface and gently knead a couple of times. Divide the dough in 2 and gather the dough pieces in a rough square. Lightly sprinkle flour over top of the dough and wrap in plastic wrap or place in a zip lock bag. Place in the freezer for about 30 minutes. While the dough is chilling, make the filling.
Make the filling:
In a bowl of an electric mixer fitted with a whisk attachment (you can also use a hand mixer or a whisk and some serious arm strength), combine the cream cheese and guava paste. Mix over medium/high speed until combined. Turn off the mixer, scrape to combine, and add the confectioners' sugar. Turn the mixer speed to low and add the vanilla extract. Mix on low speed until the confectioners’ sugar is mostly combined with the guava/cheese mixture. Raise the speed to medium/high and mix until the ingredients are fully combined and the filling is light and fluffy. If you prefer a stiffer filling, feel free to add more confectioners’ sugar. A good rule would be to add 1 tablespoon at a time until you get the desired thickness. Wrap the surface of the bowl in plastic wrap and place in the refrigerator until ready to use.
Bake the pie shells:
Lightly spray 2 24-cavity mini muffin tins with cooking spray and preheat the oven to 350 degrees Fahrenheit.
Remove one piece of dough from the freezer. Turn the dough onto a lightly floured surface and roll out a 9x12 inch rectangle to about 1/8 of an inch in thickness. If you are unable to get a 9x12 inch rectangle get as close as you can. Trim the very edges of the dough with a pizza cutter or a knife to make a nice rectangle (If you end up with a square I won’t judge you)
Using a 2 inch fluted biscuit cutter or a round cutter, working quickly cut dough circles. Gather the excess dough into a rough square and freeze for a few minutes before you re-roll and cut once more. Place the cut dough circles into the muffin cavities and press lightly to cover the entire cavity, lightly pressing on the bottom. It doesn’t have to be perfect. This dough is very buttery, so you may not end up with pretty mini pie shells and that’s OK. Plus they’ll get filled later. Place the muffin tin in the freezer to chill. Repeat with the second piece of dough. Place the second muffin tin in the freezer to chill for at least 5 minutes.
Remove both mini muffin tins from the freezer. Brush the surface of the dought with beaten egg and place in the oven. Bake for 10 to 12 minutes until the tops are golden brown. Remove from the oven and allow to cool completely before filling.
To fill the mini pie shells:
Remove the filling from the refrigerator. Fill a piping bag fitted with a round or star tip with the guava cheese filling and pipe that situation as high as you’d like. Alternatively, you can spoon the filling into the mini pie shells using a cookie scoop or a teaspoon. Top with little guava pieces for garnish.