Roll 2-3 room temperature lemons on your counter to loosen juice in side ; Using a fine grater , grate lemon rind to get your zest before you cut the lemons , cut lemons and squeeze the lemons through a strainer to catch any seeds and lemon pulp.
In the top of a double broiler combine the eggs , sugar and then add the FRESHLY squeezed lemon juice.
Cook over simmering hot water ( WHISKING the whole time so it don't curdle ) for 10 minutes or longer ; it will become the consistency of thick pudding. You can use a thermometer and measure the curd , it should reach around 170°.
Note: It may take longer than the 10 minutes of stirring time , try getting the curd to the same simmering temp before you start timing , because sometimes when you use a double broiler its hard to get it up to the proper temp.( I don't always use a D.B. just don't bring it to a boil or let it scorch if using direct heat.)
Remove from the heat and whisk the butter into the curd until butter is dissolved ; Mix in the shredded lemon zest.
Let cool ; it will continue to get thicker as it cools.
Pour into a small jar and seal tightly or cover with plastic wrap ( laying on top ; touching the curd ) so it does not form a skin like pudding does when its not covered.
Refrigerate and can be stored for up to 7 days.
Curd can be used in filling in donuts , jelly rolls , cakes , ect...