Damson Plum Pinwheels

Whisk together flour, salt, and cardamom in a bowl.
Beat together butter, cream cheese, and granulated sugar in a large bowl with an electric mixer at medium-high speed until fluffy, about 3 minutes, then beat in yolk and vanilla. Reduce speed to low and add flour mixture in 3 batches, mixing until dough just comes together. Quarter dough and flatten each piece into a block, then chill, wrapped in plastic wrap, until firm, 2 to 3 hours.
Put oven rack in middle position and preheat oven to 350°F. Line 2 large baking sheets with parchment paper.
Keeping remaining 3 pieces chilled, roll out 1 piece of dough 3/4 inch thick on a lightly floured surface with a lightly floured rolling pin, then trim edges to make a 12- by 8-inch rectangle. Chill scraps, wrapped in plastic wrap. Cut rectangle into 2-inch squares with pastry wheel.
Make a 1-inch-long cut in 1 square from tip of each corner in toward center with a small knife, halving each corner. (You will have 8 points.) With tip of knife, lift every other point and gently fold into center, overlapping ends slightly. Press center lightly to form a small well. Put 3/4 teaspoon of jam in center and transfer with a spatula to a lined baking sheet. Repeat with remaining squares, arranging pinwheels 2 inches apart on sheet.
Brush cookies with lightly beaten egg white and sprinkle with decorative sugar.Bake until edges are pale golden, 10 to 15 minutes. Cool 5 minutes on sheet, then transfer from parchment to a rack to cool completely.
While first batch is baking, form pinwheels with another piece of dough. Bake in same manner, then gather scraps and chill until dough is firm enough to reroll, 15 to 20 minutes. Make more cookies with remaining 2 pieces of dough and with scraps in same manner (reroll scraps only once).