Step #1 In a medium bowl, whisk the flour, baking powder & salt.
Step #2 In another medium bowl, using an electric mixer, beat the butter & sugar at lowish speed until mixd.
Step #3 Beat in the egg yolks & vanilla.
Step #4 Add dry ingredients & beat at lowish speed until a soft dough forms.
Step #5 Halve the dough & form into logs.
Step #6 Wrap the dough in plastic wrap & put in the fridge until firm, at least 1 hr.
Step #7 Preheat oven to 350 degrees.
Step #8 Butter an 8 1/2" x 11" glass baking dish.
Step #9 Working over the baking dish, coarsely shred 1 log of dough on the large holes of a box grater, evenly distributing the dough into the baking dish.
Step #10 Do NOT pat or press the dough.
Step #11 Using a spoon, dollop the preserves over the dough & carefully spread it in an even layer.
Step #12 Grate the second log of dough on top.
Step #13 Using a rubber spatula, tuck in any shreds of dough sticking to the side of the baking dish to prevent them from burning.
Step #14 Bake the shortbread on the bottom rack of the oven for about 35 mins, covering it with foil wrap halfway through baking.
Step #15 The shortbread is done when the pastry is golden all over.
Step #16 Let cool completely, then cut into 24 bars.
Step #17 Dust the top this with powdered sugar & serve.