3 tbsp. potato starch
1 c. cold milk
2 egg yolks
1 1/2 c. sifted confectioners' sugar
1 1/4 c. (2 1/2 sticks) unsalted butter, softened
1 tbsp. vanilla extract

Place the potato starch in a small heavy saucepan. Gradually whisk in the milk until smooth and then whisk in the egg yolks and 1 cup of the confectioners' sugar. Place over moderately low heat and, stirring constantly, cook until the mixture becomes very thick and beings to boil, about 10 minutes. Turn this buttercream base into a shallow dish. Cover with plastic wrap placed directly on the surface and cool to room temperature.

In a large bowl combine the butter and remaining 1/2 cup confectioners' sugar; beat until light and fluffy, 1 to 2 minutes. Using an electric mixer with a very strong motor, preferably an upright model, beat in the buttercream base bit by bit, beating until smooth. Beat in the vanilla. Use to frost cakes at room temperature; the frosting will set when chilled.

ORANGE BUTTERCREAM:

Beat in 2 teaspoons grated orange zest and 2 tablespoons orange liqueur.

Makes 3 1/2 cups.