Prep. Preheat your oven to 350°F (175°C). Then, line two baking sheets with parchment paper and set them aside.
Make the cookie dough. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar at medium speed until light and fluffy, for about 3 minutes. Add the egg and vanilla extract and mix on low speed until fully incorporated.
Combine the dry ingredients. In a separate bowl, whisk together the flour and salt. Gradually add the dry ingredients to the butter mixture, mixing at low speed until dough forms. The dough may be slightly crumbly but should hold together when pressed; if it doesn't, add a small sprinkle of water.
Roll out and cut. Roll out the dough to a thickness of about ¼ inch on a lightly floured surface. Then, use a large heart-shaped cookie cutter to cut out cookies. Reroll the scraps to make more cookies as needed. Place half of the cookies on your prepared baking sheets. Using a smaller heart-shaped cookie cutter, cut out the centers of the remaining cookies to create a heart-shaped "window."
Bake. Bake the cookies for 10-12 minutes or until the edges are lightly golden. Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
Assemble. Spoon a small amount of jam onto the base of each whole heart cookie, spreading it evenly but avoiding the edges to prevent overflow. Dust the top cookies (the ones with the "windows") with powdered sugar. Gently place the dusted top cookies over the jam-covered ones, aligning the edges carefully.
Serve. Store any leftovers in an airtight container at room temperature for 3-4 days.