How to Make It
In a medium bowl, whisk the flour, baking powder and salt. In another medium bowl, using an electric mixer, beat the butter and sugar at low speed until combined. Beat in the egg yolks and vanilla. Add the dry ingredients and beat at low speed until a soft dough forms. Halve the dough and form into logs. Wrap the dough in plastic and refrigerate until firm, at least 1 hour.
Preheat the oven to 350° and butter an 8 1/2-by-11-inch glass baking dish. Working over the baking dish, coarsely shred 1 log of dough on the large holes of a box grater, evenly distributing the dough in the baking dish. Do not pat or press the dough. Using a spoon, dollop the preserves over the dough and gently spread it in an even layer. Grate the second log of dough on top. Using a rubber spatula, tuck in any shreds of dough sticking to the side of the baking dish to prevent them from burning.
Bake the shortbread on the bottom rack of the oven for about 35 minutes, covering it with foil halfway through baking. The shortbread is done when the pastry is golden all over. Let cool completely, then cut into 24 bars. Dust the tops with confectioners' sugar and serve.
Any preserves could stand in for the raspberry jam in these versatile, buttery bars. For a tea party, cut the pastry into 1-inch bites and arrange decoratively on a platter.