Step 1

Thoroughly whisk together the sugar and egg yolks in a medium saucepan. Add the lemon juice, zest and salt.

Step 2

Place the butter in a heat proof bowl. Put a fine mesh sieve over the bowl and keep it near the stove.

Step 3

Heat the yolk/juice mixture over medium low heat, stirring constantly. You must be sure to keep stirring across the entire bottom and corners of the pot so the custard does not have a chance to stick. The mixture will start out opaque with a fine white foam over the surface. As it heats up the curd will thicken, become more translucent and the foam will disappear.

Step 4

Cook until the curd will coat the back of a wooden spoon and just begins to boil. Don't allow it to come to a rolling boil. One or two bubbles is all you need to see.

Step 5

Immediately remove from the heat and pour through the sieve over the butter.

Step 6

Stir until the butter is melted and thoroughly incorporated. Cover the bowl with plastic wrap and cool a bit before putting in the refrigerator.

Step 7

Refrigerate until completely cooled.