Melt butter in a small bowl. Use a pastry brush to thoroughly coat the cups AND top surface of a muffin tin. Set aside.
Roll pie crust out to 1/8″ thickness. Cut out 12 circles with a large biscuit cutter (or cocktail glass, takeout container lid, or anything of the right size). Use a smaller biscuit cutter or a large drinking glass to cut 12 smaller pie crust circles (heart cutouts optional).
Pop the smaller circles in the fridge. Use one large circle to line each muffin cup, pushing firmly to remove air bubbles and to create nice high sides. Prick thoroughly with a fork, bake for 10 minutes, then remove from oven.
Meanwhile, place berries and sugar in a saucepan over medium heat. Cook for about 10 minutes or until berries are soft. Remove from heat and mash berries with a potato masher or fork.
Mix cornstarch with a few teaspoons of water or cooked berry liquid. Stir this mix into the berries, along with the lemon zest and salt.
Fill muffin cups to the top with berries. Brush the rim of each cup with egg wash and add a top crust, doing your best to seal the top and bottom crusts together with fingers or the back of a spoon. If you didn’t cut shapes into the top crusts, use a sharp knife to cut a few slits in each one.
Brush top crusts with egg wash and sprinkle with sugar. Bake for 25 minutes, or until crusts are golden but before the berry mix bubbles over.
Remove from oven. While still slightly warm, go around the edges with a butter knife to make sure the pies aren’t sticking to the muffin tin. Then use a silicon spatula to gently lift mini pies to a wire rack. Gloat.