Preheat your oven to 375 degrees, and line two baking sheets with parchment paper.
In the bowl of your electric mixer on medium-high, beat together the butter and sugar until light and fluffy, about 2-3 minutes. Beat in egg yolk and vanilla. Next add the flour, salt and poppy seeds, mix until the dough starts to stick together.
Form the dough into 1" balls and arrange them on the baking sheets. Flatten each ball slightly with your thumb, leaving 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 ½ tsp of lemon curd.
Bake for an additional ⅔ minutes, until the curd is set and the edges of cookies are lightly golden. Transfer to wire racks to cool completely, serve.