You can transform the flavor of this filling into chocolate-mint or mocha by substituting 1/4 teaspoon pure peppermint extract or 1/2 teaspoon pure coffee extract for the vanilla.

Ingredients

　　4 1/2 ounces semisweet chocolate, chopped (1 cup)

　　3/4 ounce unsweetened chocolate, chopped

　　2/3 cup (half of one 14-ounce can) sweetened condensed milk

　　1 tablespoon unsalted butter

　　1 teaspoon pure vanilla extract

　　Melt chocolates in a heatproof bowl set over (but not touching) simmering water, stirring occasionally. Add condensed milk, butter, and extract; stir to combine. Continue to stir over simmering water until all ingredients are smooth, 2 minutes. Remove from heat, and set aside to cool slightly.

　　Reprinted with permission from Martha Stewart's Baking Handbook by Martha Stewart. © 2005 Clarkson Potter