Fill the bottom of a double boiler with 1 to 2 inches of water. You can also make your own double boiler using a saucepan and a bowl that fits just inside the pan. Make sure that the bowl does not touch the water in the pot below. Place the pan on high heat and bring to a boil.

Reduce to a simmer over medium-low heat.

Place the top of the double boiler on top. Whisk together egg yolks, lemon zest, lemon juice, and salt. Continue to whisk as the curd cooks. By whisking constantly, you'll prevent the egg yolks from curdling. Cook until the mixture thickens like a thicker cream sauce. It should easily coat the back of a wooden spoon at this point. It will take about 10 minutes.

Remove from heat and whisk in the butter until melted.

Optional step: pour the curd through a fine mesh sieve for the creamiest lemon curd!

Pour into a bowl and cover with plastic wrap, pressing the plastic wrap to adhere to the top of the curd. This will prevent a film from forming.

Let cool completely. The curd will thicken as it cools. Remove plastic wrap and store in an airtight container or jar.

Refrigerate for up to 10 days.