Whisk the eggs and sugar in a saucepan until smooth, then place pan over a low heat. Add the butter, lemon juice and zest and whisk continuously until thickened. Strain through a sieve into a sterilised jar. Lemon curd keeps, covered, in the fridge for 2 weeks.
In a mixing bowl, whisk together the flour, salt and lemon zest. Cream the butter and sugar until light and fluffy with a hand mixer. Add egg yolk and lemon extract. Beat until combined. Sift the flour mixture into the creamed butter mixture and mix until the dough starts to come together. Dump on a floured board and roll together into a flat disk. Wrap in plastic and chill for 30 minutes.
Preheat the oven to 180C/350F. Line a large baking tray with parchment paper. Roll the dough into 1-inch balls and lightly roll it in poppy seeds. Place on the prepared baking tray and press a light indentation into the top of each with your finger. Drop 1/2 teaspoon of lemon curd into each indentation. Bake for 15-18 minutes until lightly browned. Remove from the oven and place on a wire rack to cool.