Shortbreads: In a separate bowl whisk the flour with the salt. Set aside.
In the bowl of your electric mixer (or with a hand mixer), cream the butter until smooth (about 1 minute). Add the sugar and beat until smooth (about 2 minutes). Beat in the vanilla extract. Gently stir in the flour mixture just until incorporated. Flatten the dough into a disk shape, wrap in plastic wrap, and chill the dough for at least an hour.
Preheat oven to 350 degrees F (177 degrees C) with the rack in the middle of the oven. Line two baking sheets with parchment paper.
On a lightly floured surface roll out the dough until it is about 1/4 inch (1 cm) thick. Using a 2 to 3 inch (5 to 7.5 cm) cookie cutter (round, square, heart, etc.) cut out the dough. Place the cookies about 1 inch (2.54 cm) apart on the prepared baking sheets. Use a smaller cookie cutter (3/4 - 1 inch (2.54 cm)) to cut out the centers of half of the cookies on the baking sheet. (You will be sandwiching two cookies together and there will be a small 'window or cut out' in the top cookie so you can see the jam underneath.) Place the unbaked cookies, on the baking sheet, in the refrigerator for about 15 minutes. This will firm up the dough so the cookies will maintain their shape when baked. Bake for about 10 minutes, or until cookies are lightly browned. Cool on a wire rack.
To Assemble Cookies: Place the cookies with the cut-outs on a wire rack and dust the tops with the confectioners' (powdered or icing) sugar.
On the bottom surface of the full cookie (top of cookie will face out) spread with about a 1/4 - 1/2 teaspoon of jam. Sprinkle with a little grated white chocolate. Place the cut-out cookie on top and gently sandwich them together, making sure not to smug the confectioners' sugar. Using a small spoon, fill the cut-out with a little more jam.