In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg, vanilla and lemon extract if using and mix again.
Add the flour, baking powder, poppy seeds and lemon zest. Fold until combined. The dough should be fairly solid at this stage (it ought to almost form a ball, albeit a very soft one!).
Cover and chill in the fridge at least 30 minutes.
Preheat the oven to 200C/180C Fan/Gas 6. Line 2-3 large baking trays. Roll into about 10 balls (use a tablespoon to scoop balls of dough). Place on lined baking sheets.
Place an indentation in each ball using your thumb/a teaspoon. Dollop a teaspoon of lemon curd into each cookie centre. Wrap the dough to ensure the curd is totally sealed in.
Bake 10-12 minutes, until golden and just crisping up at the edges (they continue to cook a little once removed). Let cool on a wire rack!
