Place egg yolks in a large heat-safe bowl. Set aside.

In a medium-size saucepan, continuously stir the sugar, lemon zest, lemon juice and cubed butter with a heat-proof spatula over medium-low heat.

Once the mixture comes to a simmer, temper the egg yolks by SLOWLY pouring the hot liquid into the bowl of egg yolks while whisking constantly, until fully combined.

Pour mixture back into the saucepan and continue to cook low and slow. Stir continuously with a spatula or whisk until the mixture thickens and bubbles start to come to the surface; about 15 minutes. Do not stop stirring, and don't rush it. It should take the full time to cook and thicken properly.

Once the curd is thick and starting to boil, remove the pan from the heat, and push the curd through a fine mesh sieve into a clean container. Cool to room temperature. Then place the curd in the refrigerator until ready to use. The lemon curd will continue to thicken as it cools. Keep refrigerated.