In a mixer fitted with a paddle attachment (or using a hand mixer), cream the butter until soft and fluffy. Add the egg yolks and mix well. In a medium bowl, stir together the granulated sugar, flour, baking powder, and salt. Add to the butter/egg yolk mixture and mix just until incorporated and the dough starts to come together. Turn the dough out onto a floured work surface and form into 2 balls. Wrap each ball in plastic wrap and freeze at least 2 hours or overnight, or as long as a month.
Preheat the oven to 350 degrees F.
Remove 1 ball of dough from the freezer and coarsely grate the frozen dough into the bottom of a 9 by 12-inch baking pan or a 10-inch tart pan with a removable bottom. Make sure the surface is covered evenly with shreds of dough. With a spoon or spatula, spread the jam over the surface, to within 1/2-inch of the edge all the way around. Remove the remaining dough from the freezer and coarsely grate it over the entire surface. Bake until light golden brown, 30 to 40 minutes. As soon as the shortbread comes out of the oven, dust with confectioners' sugar. Cool on a wire rack, then cut in the pan with a serrated knife.