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

In a small bowl, rub the lemon zest and dried lavender into the granulated sugar with your fingertips until fragrant.

In a large bowl, cream together the softened butter and the infused sugar mixture until light and fluffy, about 2-3 minutes.

Beat in the egg yolks, one at a time, until fully incorporated.

In a separate bowl, whisk together the flour and salt. Gradually add this to the wet ingredients, mixing on low speed just until combined. Do not overmix.

Shape the dough into 1-inch balls and place them on the prepared baking sheets, spaced about 2 inches apart.

Using your thumb or the back of a small spoon, make an indentation in the center of each cookie.

Fill each indentation with about 1/2 teaspoon of jam or curd.

Bake for 12-15 minutes, or until the cookies are set and the bottoms are lightly golden.

Allow cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.