In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and ½ cup of the powdered sugar on medium speed until light and fluffy, about 5 minutes.
Mix in the vanilla until combined.
Sift in the flour and salt. Mix on low speed until just incorporated.
Gather and form a dough. Cover with plastic wrap and refrigerate 30 minutes.
Preheat oven to 325 ºF (165 °C). Line two large cookie sheets with parchment paper (I love the big Nordic Ware cookie sheets that you can get on Amazon).
On a lightly floured surface, roll out the dough to ¼-inch thickness. Cut out circles with 2-inch cookie cutter. Re-roll scraps to cut out as many circles as possible. You should get about 40 circles. With a tiny heart, star or diamond cookie cutter, cut out centre window in 20 of the circles. Place cookies on prepared cookie sheets, spacing them out.
Bake 15 to 18 minutes. Let the edges brown just a touch. Not too much! Let cool completely.
How to assemble shortbread sandwich cookies with jam
Dollop a teaspoon of jam on the 20 "full circle", bottom side of the cookies. Top with the cookies with a centre cut-out window.
Dust lightly with powdered sugar.
Notes
Recipe makes approximately 40 cookies, but if you sandwich them together with a filling, that would make roughly 20 sandwich cookies once assembled.
This recipe comes from Oh Sweet Day by Fanny Lam (available on Amazon)