Chocolate Sour Cream Frosting:
1.Heat the chocolate in a double boiler or medium heatproof bowl set over (not in) a saucepan of simmering water, stirring often, until melted and smooth. Remove from heat and let cool, 10 to 15 minutes.
2.Using an electric mixer, beat the butter on high until light and fluffy, 3 to 5 minutes. Reduce mixer speed to low. Gradually add the sugar and beat until smooth, scraping down the sides of the bowl as necessary. Add the chocolate, sour cream, and salt and beat to combine.

Chocolate Ganache Frosting:
1.Place the chocolate in a large heatproof bowl. In a medium saucepan, bring the cream to a bare simmer. Pour the hot cream over the chocolate, add the salt, and let stand for 2 minutes. Whisk until melted and smooth. Let cool, stirring occasionally, until thick and spreadable, 4 to 5 hours. (Note: Don’t try to speed the cooling process by putting the frosting in the refrigerator; it will become lumpy.)

Vanilla Frosting:
1.Using an electric mixer, beat the butter on high until light and fluffy, 3 to 5 minutes. Reduce mixer speed to low. Gradually add the sugar, then the vanilla and salt, and beat until smooth, scraping down the sides of the bowl as necessary

3 FROSTING VARIATIONS:
Follow the vanilla frosting recipe above with these modifications:

-Lemon Frosting:
Beat in 2 tbs. finely grated lemon zest with the butter. Omit the vanilla extract.
-Coffee Frosting:
In a small bowl, stire together 1 Tbs. instant espresso powder and 1 Tbs. hot water until the powder is dissolved. Add to the frosting along with the vanilla and salt.
-Caramel Frosting:
Add ½ cup prepared dulce de leche to the frosting along with the vanilla and salt.