Preheat the oven to 350 degrees F (175 degrees C).

Cream sugar and butter with an electric mixer until fluffy, about 5 minutes. Mix in egg and vanilla extract. Mix flour, baking powder, and salt together in a separate bowl and add to butter mixture. Mix well.

Put a handful of the dough between two sheets of parchment paper and roll to desired thickness. Repeat with the rest of the dough. Put the rolled dough, including the parchment paper, on a cookie sheet and refrigerate for 10 minutes or longer. Repeat the process with scraps after cutting cookies.

Cut 24 heart-shaped cookies of the same size from the dough. Use a smaller heart-shaped cookie cutter to cut a second hole in half of the cookies; this will be the window where the jam will show through. Transfer the cut cookies to a baking sheet.

Bake in the preheated oven until golden, 8 to 10 minutes. Transfer the baking sheet to a wire rack to allow to cool, 20 to 30 minutes. Spread about 1/2 teaspoon of jam into the centers of the cookies without the heart-cut centers. Sprinkle powdered sugar on the rest of the cookies with the heart-cut centers, and mount onto the cookies with jam.