Lemon Curd:
To avoid a metallic aftertaste in the lemon curd, use non-metal utensils and cookware, such as a glass or ceramic bowl, a rubber spatula, and a non-metal saucepan.
In a bowl, combine the sugar and lemon zest, rubbing the zest into the sugar with your fingertips to release the essential oils.
Add the egg yolks and salt to the lemon-sugar mixture and whisk until the mixture is slightly fluffy and pale yellow.
In a saucepan, heat the lemon juice over medium-high heat until it just begins to boil.
Gradually pour the hot lemon juice into the egg yolk mixture, whisking constantly to prevent curdling.
Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens and coats the back of a spoon (about 4-5 minutes). Do not let it boil.
Remove from heat and stir in the cubed butter until fully melted.
Strain the lemon curd through a fine mesh sieve to remove the zest for a smooth finish (optional).
Transfer the lemon curd to a bowl and cover with plastic wrap, pressing the wrap directly onto the surface to prevent a skin from forming. Let it cool completely to room temperature. (You can make the lemon curd ahead of time and refrigerate it until needed.)
Lemon Cookie Dough:
In a large bowl, combine the sugar and lemon zest, rubbing the zest into the sugar with your fingertips to enhance the lemony aroma.
Add the melted butter, eggs, lemon juice, and vanilla, whisking until well combined.
In a separate bowl, whisk together the gluten-free flour blend, xanthan gum, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, stirring with a wooden spoon or rubber spatula until a smooth, batter-like dough forms. The dough will be very soft and sticky, so don’t add more flour.
Chill the dough in the refrigerator for at least 2 hours or overnight.
Assembling & Baking the Cookies:
Preheat your oven to 350ºF (180ºC) and line two large baking sheets with parchment paper.
Using a 2-tablespoon cookie scoop, portion out the dough and drop each scoop into a bowl of powdered sugar. Roll the dough in the sugar, then shape it into a ball with your hands.
Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart.
Use a ½-tablespoon measuring spoon to create an indent in the center of each dough ball, pressing only halfway through.
Fill each indent with about 1 teaspoon of lemon curd.
Bake one sheet at a time for 9-12 minutes, until the cookies have spread, cracked around the edges, and the lemon curd is slightly puffed but not bubbling.
Allow the cookies to cool on the baking sheet for 5-10 minutes before transferring them to a wire rack to cool completely.
These cookies are delicious at room temperature but are especially delightful when chilled. Store leftovers in the fridge.