Combine flour, baking powder, and salt in a bowl.

In a mixer bowl, beat butter and sugar on medium-high until fluffy, about 3 minutes. Add egg and beat until smooth. Reduce speed to low; add flour mixture, lemon juice, and zest. Beat, scraping down sides, until combined. Shape dough into a disk. Wrap in plastic and refrigerate 3 hours.

Preheat oven to 350 degrees F. Line several baking pans with parchment paper. Roll out dough on lightly floured surface until it's inch thick. Cut out 1-inch heart shapes and place on prepared pans. For half the hearts, use a -inch cookie cutter to cut a heart from center. Reroll these hearts and other scraps; repeat until all dough is used. Bake until golden, 8 to 10 minutes. Cool on a wire rack.

Spread jam on whole cookies. Dust cutouts with powdered sugar. Sandwich together.