1ift flour, baking powder, and salt into a large bowl. Cream butter and granulated sugar, with a mixer on medium speed, until pale and fluffy. Beat in yolks and vanilla until well combined. Add flour mixture in 3 additions, alternating with cream and beating well after each addition. Shape dough into 4 disks, wrap each in plastic, and refrigerate for at least 3 hours (or up to 3 days).
2On floured parchment, roll 1 disk of dough into a 1/8-inch-thick rectangle. Repeat with remaining disks. Stack sheets of dough and parchment, and refrigerate for 1 hour or freeze for 30 minutes.
3Spread raspberry jam on 1 sheet of dough, blueberry jam on another. Top each with a plain sheet of dough. Refrigerate for 1 hour or freeze for 30 minutes.
4Preheat oven to 350 degrees. Cut out cookies using a 2 1/2-inch star-shaped cutter, and space 1 inch apart on parchment-lined baking sheets. Brush with cream, and sprinkle with sanding sugar. Refrigerate for 30 minutes or freeze for 15 minutes.
5Bake, rotating sheets halfway through, until golden, 16 to 20 minutes. Transfer cookies to wire racks, and let cool completely. If desired, bake dough scraps.