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

In a medium-sized bowl, whisk together the flour, baking powder, and salt.

In a large bowl, cream together the softened butter and sugar until light and fluffy.

Add the egg, lemon zest, lemon juice, and vanilla extract to the butter mixture. Mix until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing until a dough forms.

Roll the dough into small balls, about 1 inch in diameter, and place them on the prepared baking sheet. Slightly flatten each ball with the back of a spoon.

Bake for 8-10 minutes, or until the edges of the cookies are golden brown. Remove from the oven and let them cool on a wire rack.

While the cookies are cooling, prepare the lemon curd filling. In a saucepan over medium heat, whisk together the lemon juice, lemon zest, sugar, and eggs until smooth.

Continue to cook the mixture, whisking constantly, until it thickens and reaches a custard-like consistency.

Remove the curd from the heat and stir in the butter until melted and smooth. Let it cool slightly before using.

Once the cookies have cooled, use a spoon or piping bag to fill the center of each cookie with the lemon curd.