How to Make It
Whisk together the flour, baking soda, and salt in a bowl.
Beat the butter and sugar in a bowl with an electric mixer on medium-high until light and fluffy, 2 to 3 minutes. Beat in the egg yolks and then the sour cream, scraping down the sides of the bowl as necessary, until combined. Beat in the vanilla. Reduce mixer speed to low and gradually add the flour mixture; stir until just combined. Shape into 2 1-inch-thick disks, wrap in plastic wrap, and refrigerate for at least 2 hours and up to 3 days.
Heat oven to 350° F.
Line 2 baking sheets with parchment.
Roll the dough 1/8 inch thick on a floured surface. Using a lightly floured 11/2- or 2-inch round cookie cutter, cut the dough into circles, flouring the cutters and rerolling the scraps as necessary. Cut smaller decorative holes in the centers of half the cookies. Refrigerate until firm, at least 30 minutes.
Combine the egg whites and 1 teaspoon water in a small bowl. Spread the jam on the uncut cookies (about 1/2 teaspoon each), dividing evenly and leaving an 1/8-inch border all around. Brush the borders with the egg wash and top with the cut cookies. Use a fork to crimp the edges.
Place the cookies 1 inch apart on the baking sheets. Bake, rotating the sheets halfway through, until golden brown, 13 to 15 minutes. Let cool slightly on the baking sheets, then transfer to a wire rack to cool completely.