For the Lemon Curd:

1. Add the lemon zest and juice to a medium saucepan. Add the sugar and set the pan over medium heat.
2. Stirring frequently, bring the mixture to a simmer and stir until the sugar is completely dissolved.
3. Reduce the heat to low, making sure the mixture is below a simmer, and whisk in the butter and eggs.
4. Stir almost constantly, keeping the mixture below a simmer, until it thickens, about 8 minutes.
5. Strain the curd through a fine-mesh strainer into a bowl. Cover with plastic wrap, pushing the plastic wrap directly onto the surface of the lemon curd, and then cool completely. I like to keep the lemon curd in the fridge until it’s time to fill the cookies.

For the Cookie Dough:

1. Cream the butter and sugar together in the bowl of a stand mixer (you can also use a hand mixer) until creamy and smooth.
2. Add in the lemon zest, lemon extract, vanilla extract, and egg yolks. Mix to combine.
3. In another bowl, whisk together the flour, cornstarch, salt, and poppy seeds.
4. Add the dry ingredients to the wet and mix until the dough comes together. If it’s a little crumbly, just press the dough with your hands until it comes together.
5. Using a 1-tablespoon scoop, scoop out balls of the dough and roll them smooth between your hands.
6. Roll the dough balls through the granulated sugar and then transfer them to a large baking sheet lined with parchment paper.
7. Using the back of a ½-teaspoon measuring spoon, gently press a divot into the center of each cookie. If the edges crack apart, just gently press them back together with your fingers.
8. Once all the cookies have been pressed and formed, transfer the baking sheet to the freezer for 20 minutes.
9. During this time, preheat the oven to 375°F.
10. Remove the cookies from the freezer and transfer straight to the oven. I did this in two batches to ensure there is enough room between the cookies. Bake for 10-12 minutes or until the cookies are lightly golden around the edges and set.
11. Remove the cookies from the oven and allow them to cool completely on the tray.
12. Once they are cooled, fill the cookies with the lemon curd, using roughly 1 teaspoon for each cookie.
13. Allow them to set completely before storing – for me this took about 30 minutes of sitting at room temperature.