In a small saucepan stir the lemon juice, water and granulated sugar.
Bring to a gentle boil and remove from heat to cool.
Combine egg yolks, cooled syrup and grated lemon peel in a double boiler.
With a wire whisk, stir mixture until it thickens enough to form a ribbon as it falls from the whisk.
Remove from heat, and fold in the butter. Cool.
Spoon mixture into baked tartlets or use as a spread for toast.
Keeps refrigerated in a glass jar for 1 week and frozen for several months.