Preheat oven to 400 F. Line 2 baking sheets with parchment paper.
Cut plums on half, remove the pits and thinly slice.
Thaw puff pastry, following the instructions on the package.
Dust the counter with some flour, unfold the puff pastry, roll slightly and cut each piece (there are 2 in a package) into 9 squares.
Mark another square, about 1/2 inch from the outside over the puff pastry, scoring a square border on the inside of the pastry. Using a fork, make some dots over the inside square. This is where the sliced plums will go, only covering the inside square.
Brush the outside border with egg wash. Egg Wash brings out our pastry’s golden color even more!
Sprinkle each tart with sugar on top and bake for 15-18 minutes at 400 F.
Take out of the oven, let the pastry cool for 5 minutes and remove from the pan. Dust with powdered sugar (optional).