Preheat your oven to 375°F, and line two baking sheets with parchment paper or baking mats.
In a medium bowl, beat together the butter and sugar until light and fluffy, about 2-3 minutes with an electric mixer.
Beat in egg yolk and vanilla. Next add the flour, salt and poppy seeds, mix until the dough begins to stick together and form a ball.
By hand, roll the dough into 1-inch balls and arrange them on the baking sheets. Flatten each ball slightly with your thumb to leave an indentation in the center of each cookie.
Bake for 10 minutes. Remove the baking sheet from oven (if the indentations look shallow push them down slightly). Fill each thumbprint with lemon curd.
Bake for an additional 2 minutes. This will set the curd. Transfer to cooling racks and cool.