For the pastry: Roll the short crust pastry 1/4-inch thick and cut into 6 (6-inch) rounds.
Refrigerate the pastry rounds for at least 30 minutes. Cut 5 evenly spaced 1-inch slits around the edge of each pastry round (perpendicular to the edge).
Whisk together the egg yolk and water to make an egg wash. Brush the edge of the pastry with egg wash.
For the filling: Toss the plums with the brown sugar and cardamom. Pile an equal amount of plum mixture in the center of each pastry round. Fold the pastry around the plums, overlapping the pastry at the slits, and press firmly to seal. Place the galettes on a parchment paper lined baking sheet. Brush the outside of the pastry with egg wash.
For the streusel topping: Combine the flour, sugar and cinnamon together. Using your fingers, cut the butter into the flour mixture until it resembles cornmeal. Sprinkle the streusel on top of each galette.
Bake for 20 minutes. Reduce the oven temperature to 350 degrees F and bake for 20 minutes longer. Serve warm.
Short Crust Pastry:
Yield: 2 pounds
Sift the flour and salt into a large mixing bowl. Using a cheese grater, grate the butter and shortening into the flour. Combine the mixture with your fingers until it resembles cornmeal. Add the ice cold water and mix with a spoon, pulling the water into the flour until a dough forms.
With your hands, shape the dough into 2 balls and flatten them into disks. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes. Store in the refrigerator for up to 4 days and in the freezer for up to 1 month.