Cook bacon: on a fry pan over medium heat, cook bacon until crisp and then remove from pan on paper towel-lined plate. Crumbled when cooled down and set aside. If you aren't cooking bacon, but are using other fillings that need to be cooked, now is the time!
Make the egg filling: In a large bowl, add eggs, milk, salt & pepper and whisk until completely combined. Cover and refrigerate until later.
Preheat oven to 375°F. Grease two 24-count mini muffin pans with nonstick spray. If you only have 1 mini muffin pan, bake these in batches. Work with 1 pie dough first and while the other stays in the fridge for the next batch of quiche.
Cut your pie crust: Take your defrosted but chilled pie dough out of the fridge and onto a floured surface or a piece of parchment paper. Using a rolling pin, roll the pie crust out just a little before you start cutting. Using a 2.5-inch cookie cutter OR a ⅓ cup measuring cup, cut your small circles out and place to the side. You will need to re-roll your pie dough scraps a few times in order to get 24 for each tin. Work quickly through this, as the longer it takes the warmer the dough gets. Your first crust should give you 24 circles. Repeat with your second crust (if you have two pans). If you feel like your pie crust is getting too warm, stick it back in the fridge to chill for a little.
Place the dough rounds into the greased 24-count mini muffin pan/pans. Press the dough flat into the bottom of the tins and up the sides.
Evenly pour the cold egg filling into each unbaked crust. I used 1 tablespoon of egg per each cup to make sure they were even. Then top evenly with bacon, and finish with a sprinkle of cheese.
Bake mini quiches until the center is just about set and edges are lightly browned, about 20-23 minutes. Allow them to cool for 5 minutes before removing from the pan. If greased well, the mini quiches pop right out using a butter knife or spoon to scoop them out. Serve warm or at room temperature.