1. Put the lemon peel and sugar in the bowl of a food processor. Pulse until you have a fine zest incorporated into the sugar.
2. In the bowl of a food processor, cream the butter with the lemon sugar mixture. Once combined, add the eggs one at a time. Once incorporated, add the lemon juice and salt and process until smooth.
3. Pour the mixture into a 2 quart saucepan and heat over low heat until thickened, about 10 minutes, stirring constantly. If you have a candy thermometer, the mixture will be just around 170° when it thickens. Remove from the heat and cool or refrigerate.
Source: Ina Garten