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Here are two recipes for "Custard Frosting" Maybe one of these is the one you are looking for?
Ingredients:
2 tablespoons flour
3/4 cup milk
3/4 cup butter
3/4 cup sugar
1/8 teaspoon salt
1 teaspoon vanilla
Instructions:
In a saucepan add a small amount of the milk to flour then stir to make a smooth paste. Add remaining milk. Cook, stirring constantly, over medium heat until mixture boils and thickens. Cool.
Cream butter using medium speed of electric mixer. Gradually add sugar and salt; beat well. Add cooled milk mixture. Beat until light and fluffy; add vanilla. Frosts a two layer cake.
AND
Mix 1/2 cup sugar and 4 tablespoons flour together.
Add 1/2 cup milk and 1 egg.
Cook until very thick, cool thoroughly and then add:
1/2 cup butter, 1 teaspoon vanilla, 4 to 8 tablespoons powdered sugar. Beat together until thick and creamy.