In a large bowl, whisk together the flour, cornstarch, and baking powder until well combined.

In a separate bowl, using a stand mixer or hand mixer, cream the butter and powdered sugar together until light and fluffy, about 2-3 minutes.

Add the egg, vanilla extract, and lemon zest to the butter mixture and beat until incorporated.

Gradually add the dry ingredients to the wet mixture, folding with a spatula until a soft dough forms. Wrap the dough in plastic wrap and refrigerate for 30 minutes.

Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.

On a lightly floured surface, roll out the dough to about 1/8-inch thickness. Cut into rounds using a cookie cutter.

Place a small dollop of jam in the center of each round. Fold two sides of the dough toward the center and pinch firmly to seal the jam inside.

Place the pinched cookies on the prepared baking sheet and bake for 10-12 minutes, until edges are lightly golden. Cool completely on a wire rack and dust with powdered sugar before serving.