In a small saucepan, whisk 1/2 cup milk into 2 teaspoons cornstarch. Cook and stir over medium heat until thickened and bubbly. Reduce heat; cook and stir for 2 minutes more. If you like, stir in 1 tablespoon rum or 1/2 teaspoon rum extract. Cover surface with plastic wrap. Cool to room temperature (do not stir). In a large mixing bowl, beat 3/4 cup softened butter for 30 seconds. Add 2 cups powdered sugar, 2 tablespoons unsweetened cocoa powder and 1 teaspoon vanilla. Beat with electric mixer on medium speed until light and fluffy. Add cooled milk mixture to butter mixture, half at a time, beating on low speed after each addition until smooth. Beat in an additional 5 to 6 cups powdered sugar until spreading consistency. Makes about 3 1/4 cups frosting.