To make the lemon curd filling, place the beaten eggs in a saucepan, add the sugar, butter, lemon zest, and juice and stir over a gentle heat until the butter and sugar have melted. Continue to cook, stirring all the time, until the mixture thickens—this will take about 20 minutes. Make sure that the mixture doesn’t boil during this time as the eggs will curdle. Set aside to cool.