15-Minute Chocolate-Walnut Fudge

1. Make foil sling by folding 2 long sheets of aluminum foil so that they are as wide as 13 by 9-inch baking pan (one 13-inch sheet and one 9-inch sheet. Lay sheets of foil in pan perpendicular to one another, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Grease foil and set aside.
2. Mix semisweet chocolate, unsweetened chocolate, baking soda, and salt in medium heatproof bowl until baking soda is evenly distributed. Stir in sweetened condensed milk and vanilla. Set bowl over 4-quart saucepan containing 2 cups simmering water. Stir until chocolate is almost fully melted and few small pieces remain, 2 to 4 minutes.
3. Remove bowl from heat and continue to stir until chocolate is fully melted and mixture is smooth, about 2 minutes. Stir in walnuts. Transfer fudge to prepared pan and spread in even layer with spatula. Refrigerate until set, about 2 hours. Remove fudge from pan using foil and cut into squares. (Fudge can be refrigerated for up to 2 weeks or frozen for up to 3 months.)

15-MINUTE PEANUT BUTTER FUDGE
Substitute 1 pound 2 ounces peanut butter chips for chocolates in step 2 and omit walnuts.

15-MINUTE ROCKY ROAD FUDGE
Substitute 1 cup mini marshmallows, 1 cup coarsely chopped peanuts, and 1⁄2 cup semisweet chocolate chips for walnuts in step 3.