Beat eggs with electric mixer @ medium speed for 1 minute.
Gradually add sugar and increase speed to high beating for about 5 minutes.
Reduce the speed to Low, add the butter and lemon juice and mix until just combined.
Transfer to heavy bottomed saucepan and cook over low heat, stirring constantly with a wooden spoon. DO NOT LET SIMMER OR EGGS WILL COOK!
Mixture is done when it coats the spoon so thickly that it can hold marks in it if you touch it or is @ 138F.
Transfer to clean bowl. Press plastic wrap against surface to keep from forming a skin. Put in fridge and let chill 6 hours to 2 days.
When mixture is cold beat with electric mixture at medium speed until frothy. Add the lemon zest and rum and continue beating until you get soft peaks.
Layer in desert dishes with fresh fruit in between layers.