Combine eggs, sugar and lemon juice in a bowl.
Place over double boiler and heat until the mixture coats the back of a spoon, or thickens.
Add butter and gently stir till the butter melts.
Transfer the lemon curd into a piping bag and pipe out a tablespoon (or any amount you'd like on top of your cake) of the curd to set aside. Set the rest in the chiller to cool.