Make the dough ahead of time, this will allow the lemon flavor to develop and the flour and powdered sugar to hydrate: melt the butter, then whisk in sugars and lemon zest. Add the vanilla, salt and baking soda and beat to combine. Add the egg, whisk until combined then sift in the flour and stir with a rubber spatula until you have a cohesive cookie dough. Set in an airtight container in the fridge for a few hours or overnight.
[Before you begin, see recipe notes for alternative method to shaping]
Portion the lemon curd on a parchment lined cookie sheet, about 2 teaspoons a dollop and set in the freezer for a few hours.
When you are ready to make the cookies, bring the cookie dough back to room temperature so it’s easier to shape - keep the curd in the freezer as it thaws quickly. Line two cookie sheets with parchment paper and preheat the oven to 350 F.
Portion all of the dough into 2 tablespoons and set the poppyseeds in a bowl.
Use your thumbs to flatten the dough, as pictured, to make space for the curd then place a frozen dollop of curd inside and squeeze the cookie dough around it. Shape them so a bit of curd peeks out at the top (it unveils the center and the cookies will look like my photos).
Roll the cookies in the poppyseeds to coat them. If the poppies aren’t sticking, wet your hands and try rolling again. Repeat with all the dough.
Stick the cookie tray in the freezer while the oven preheats, at least 10 minutes (this will help them from spreading too much).
Bake for 12-15 minutes, until golden on the edges and bottom. Avoid overbaking these unless you want a crunchy cookie and an overbaked curd.
Keep cookies in the fridge or at room temp - they'll last longer in the fridge!