1/2 lb. unsalted butter, softened
4 tbsp. powdered sugar

FOR CUSTARD:

3/4 c. sugar
4 tbsp. flour
1/8 tsp. salt
2 whole eggs
1/2 tsp. vanilla
2 c. milk

For the custard, mix all the above in the order given until blended thoroughly. Cook over medium heat until boiling. Cool thoroughly.

When custard is completely cool, cream softened butter with powdered sugar. Add butter mixture to custard and mix. Frost cooled cake.

Cooling completely is most important step.