Preheat the oven to 400 degrees F.
Option 1- Grease a 12-portion nonstick muffin tin with melted butter or non-stick spray.
Option 2- Line the muffin tin with paper baking cups.
Cut circles from the puff pastry sheets using a 3 1/2-inch plain cutter and transfer them to the cups in the muffin tin. Press the dough evenly into the edges of each muffin cup, lining the sides.
Whisk the milk, confectioners' sugar, egg yolks and egg together in a large glass measuring cup until silky smooth.
Pour the mixture into each crust, leaving 1/8 inch at the top. Bake in the center of the oven, approx 12 to 15 minutes, until the pastry begins to brown. The filling should be set but still jiggle and a toothpick inserted into the center of a tart comes out clean.