Make the lemon curd: In a medium heatproof bowl, combine the yolks, sugar, lemon juice, and lemon zest. Whisk until smooth.

Place the bowl over a pot of simmering water. Cook, stirring continuously with a whisk, until the mixture thickens, about 10 minutes.

Remove from heat, and strain the curd through a fine mesh strainer into a bowl. Add the butter and salt, and whisk until melted and fully incorporated.

Cover the curd with plastic wrap, pressing it directly onto the surface to prevent a skin from forming. Let it cool to room temperature for about an hour, then refrigerate until ready to use.

Make the lemon cookies: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a separate medium bowl, rub together the lemon zest and ¾ cup of the sugar using your fingertips until the mixture is fragrant and well combined.

Add the softened butter to the sugar mixture. Using a hand mixer, beat on medium speed until combined, 5–7 minutes. Add the eggs 1 at a time, beating well after each addition. Stir in the lemon juice and vanilla until smooth.

Add 2–3 drops of yellow food coloring if using, and beat until everything is incorporated.

Using a rubber spatula, fold the flour mixture into the egg-butter mixture gently until just combined. The dough will be soft and sticky — this is normal.

Cover the bowl with plastic wrap, and refrigerate for at least 3 hours or up to 24 hours to firm up the dough.

Preheat the oven to 350°F (175°C). Line 2 baking sheets with parchment paper.

Scoop about 1 tablespoon dough (approximately 20 grams), and roll into a ball. (You’ll have about 18 balls total.) Roll each ball first in the remaining ¼ cup granulated sugar, then generously in confectioners’ sugar.

Place the coated dough balls on the prepared baking sheets, leaving about 2 inches of space between each.

Use the back of a 1-teaspoon measuring spoon to make an indent in the center of each cookie dough ball. Fill each cookie with about 1 teaspoon lemon curd.

Bake for 10–12 minutes, until the cookies have spread and cracked around the edges and the lemon curd center is slightly set.

Allow the cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.