In a nonreactive saucepan over medium-high heat, whisk lemon juice and zest with six tablespoons of the sugar. Bring to a boil and set aside.

In a large mixing bowl, whisk eggs and remaining sugar thoroughly to create a smooth mass. In a slow, steady stream, add about one-quarter of the boiled lemon juice to the eggs while continuing to whisk vigorously. You are warming, or tempering, the eggs so that they don't form lumps. Pour the egg-lemon mixture into the saucepan containing the lemon juice. Return to the stove and whisk vigorously, bringing the mixture to a boil for one minute. Remove from heat once it thickens to the consistency of hollandaise sauce and place the pan in a bowl of ice. Cool lemon curd for about three minutes, to 120°F-130°F degrees.

Remove from ice bath and add butter in five increments, stirring gently with a rubber spatula until smooth. Pour into a glass container and chill in a refrigerator for several hours or overnight.

Note: When making lemon curd, thoroughly whisking the sugar with the eggs makes straining it unnecessary. Makes 2 cups.