Place the frozen raspberries and sugar in a small pot and cook over medium-high heat, stirring, until the fruit has broken down and the mixture is syrupy.
Continue to cook until thickened, about 5 minutes.
Allow the jam to come to room temperature, then refrigerate.
To make the cookies
Preheat the oven to 350 degrees F, and line a baking sheet with parchment paper.
Place the butter and sugar in a large mixing bowl, and cream together on medium-low speed until smooth.
Add the eggs, one at a time, scraping the bottom and sides of the bowl with a silicone spatula after each addition.
Stir in the vanilla, salt, and cornstarch.
Add the flour, about 1/2 cup at a time, mixing on medium-low speed after each addition. Stop adding flour when the dough gathers itself into a ball and pulls away from the sides of the bowl cleanly. (If making the dough ahead, be sure to add a little less flour.)
Roll the dough out between 2 sheets of parchment paper, to a thickness of about 3/8-inch.
Use a heart-shaped cookie cutter to cut shapes. Use a smaller, fluted heart cutter to cut windows in half the cookies.
Bake for 10-14 minutes, or until the edges are just barely beginning to turn golden.
Cool completely. Then spread the jam on the bottom heart, top with a second, open-heart cookie, and dust with powdered sugar.