Add the lemon zest, lemon juice, sugar, butter, whole eggs, and egg yolks to a medium saucepan.
Whisk to combine before placing the pan over the heat.
Cook over low to medium-low heat, whisking constantly.
Continue cooking until the butter is completely melted and the curd becomes smooth, glossy, and thick enough to coat the back of a spoon, approximately 6–10 minutes.
Remove from the heat immediately.
Pour the warm curd through a fine-mesh sieve into a clean bowl.
Let cool for about 15–20 minutes.
Cover the surface and refrigerate for approximately 2 hours, or until completely chilled and thickened.
Stir gently before using as tart filling.