Rub the lemon zest into the granulated sugar using your fingertips to release aromatic oils. Combine with melted butter and whisk thoroughly. Incorporate the powdered sugar, vanilla extract, and sea salt, mixing until smooth. Whisk in the egg until fully emulsified. Sift the baking soda and flour into the mixture and fold together using a rubber spatula to form a unified dough. Transfer to an airtight container and refrigerate for several hours or overnight to deepen flavor and hydrate the mixture.

On a parchment-lined baking sheet, place small dollops (approximately 2 teaspoons each) of lemon curd and freeze until solid.

Allow the dough to soften at room temperature before shaping. Keep the lemon curd frozen until needed, as it thaws rapidly. Preheat the oven to 350°F (175°C) and prepare two baking sheets with parchment paper. Portion the dough into 2-tablespoon balls.

Flatten each dough ball with your thumbs to create a cavity, then insert a frozen curd portion and wrap the dough around it, leaving a small opening at the top. Roll each formed cookie in poppy seeds; if necessary, moisten hands to help the seeds adhere.

Place the assembled cookies on a tray and freeze for at least 10 minutes to reduce spreading during baking. Bake for 12–15 minutes, or until the edges and base are golden. Avoid overbaking to preserve the soft center and delicate curd.

Store the finished cookies refrigerated or at room temperature; refrigeration will extend their freshness.