In a food processor, combine flour, cinnamon, sugar, and salt. Pulse. Add butter and pulse until dough resembles pea-sized bits. Add cold water in small increments and pulse until dough comes together (you may not need all the water; weather and humidity can affect the moistness of the dough). Do not overmix.

Dump dough out onto plastic wrap and refrigerate for at least 30 minutes.

Meanwhile, combine plum slices and brown sugar. Let sit for at least 30 minutes to macerate.

Preheat oven to 375 degrees Fahrenheit.

Remove dough from refrigerator. On a piece of Gefen Parchment Paper, roll out entire dough into a circle to make one large crostata, or divide dough into balls and roll out to make mini crostatas. Lay the plums in a fan-like pattern in the center of each circle. Fold tart dough around the plums.

Transfer parchment paper to baking sheet. Brush dough with beaten egg and sprinkle with sugar. Bake at 375 degrees Fahrenheit in a convection oven, or 400 degrees Fahrenheit in a regular oven until golden, about 20–25 minutes.