Nonstick cooking spray
1 pound (16 ounces) bittersweet or semisweet chocolate, finely chopped
4 tablespoons (1/2 stick) unsalted butter, cut into pieces
2 14-ounce cans sweetened condensed milk
1 teaspoon vanilla extract, divided
1 pound (16 ounces) milk chocolate, finely chopped
2 ounces white chocolate
Thoroughly coat all inside surfaces of a 9×13-inch pan with nonstick cooking spray; set aside.
Melt dark chocolate and 2 tablespoons butter in a double boiler, stirring until smooth. Remove from heat and stir in 1 can condensed milk and 1/2 teaspoon vanilla extract. The mixture should be smooth.
Scrape mixture into prepared pan. Use a rubber spatula or your fingers to coax fudge into corners and into an even layer. Allow fudge to cool to room temperature.
Repeat the above procedure using milk chocolate. Carefully spread this fudge on top of the first layer, taking care not to disturb first layer. Using the largest holes of a box grater, grate white chocolate. Sprinkle on top of fudge in an even layer. Gently press into fudge.
Refrigerate 2 hours, or until firm enough to cut. Cut into 2-inch squares. (Fudge may be refrigerated in an airtight container up to 2 weeks.)