In a large mixing bowl, beat the 1/2 cup melted butter, cocoa powder, half-and-half, and the 2 teaspoons vanilla with an electric mixer on medium speed till smooth. Slowly beat in 4 cups of sifted powdered sugar. If necessary, beat in additional half-and-half till spreading consistency. Spread over cake. Cover; chill in the refrigerator up to 24 hours. Cut just before serving. Makes 15 servings.