In a medium bowl, whisk together the flour, corn starch, and salt. Set aside.
In a stand mixing bowl, cream the butter and sugar with the paddle for 2 minutes, or until light and fluffy.
Mix in the egg and vanilla on low until just combined. Scrape down the bowl.
Mix in the dry ingredients on low until just combined and the dough starts to come together.
Divide the dough in half. Wrap each half loosley in plastic wrap, and use a rolling pin to roll them into a flat disc. Refrigerate until firm, about 30 minutes.
Once firm, unwrap the dough and place each slab in between two sheets of parchment paper. Roll the dough to ¼ inch thickness. Then transfer both slabs (still sandwiched in the parchment paper) to a baking sheet and freeze for 15 minutes.
Using a 2 ½" round cookie cutter, cut out an even number of cookies. Then gather the scrap dough, roll, and repeat. Once all the dough is cut, use a smaller, decorative cutter, to cut out the center of half of the cookies. Transfer them to a parchment-lined baking sheet.
Freeze the cutout cookies for at least one hour, or until they are completely frozen solid. Then bake them at 350 F for about 10-12 minutes, or until the tops look matte and the bottoms are very light brown.
Preheat the oven to 350 F/180 C. Bake the cookies for 10-12 minutes, or until the tops look matte and the bottom edge is very light brown. (for my oven, 11 minutes is perfect!)
Once cool, dust the tops of the cookies with the centers cut out with powdered sugar.
Then flip the bottom cookies upside down and spread about one teaspoon of jam onto the center, spreading it slightly. Sandwich the cut-out cookie on top and enjoy!