Lemon Curd

Process sugar and lemon zest in food processor until zest is dispersed. Beat egg yolks with sugar and lemon juice until well mixed, about 1 minute. Place bowl over double boiler and keep stirring as it cooks. Cook until smooth and thickened, about 8 to 10 minutes. Mixture should coat the back of a spoon. Temp should reach 180 deg. Remove from heat and stir in butter, one tablespoon at a time. Pour into jelly jars and refrigerate.