German Chocolate Sheet Cake With Rocky Road Frosting

For Cake: Preheat oven to 350 degrees F. Lightly mist a 13-by-9-inch pan with cooking spray.

Break squares of chocolate in half. Turn on a food processor fitted with the metal blade; drop the chocolate through the feed tube, one piece at a time, and process until finely grated. (Or grate by hand.)

Combine cake mix, sour cream, water, oil, eggs, vanilla and grated chocolate in a large mixing bowl. Beat with an electric mixer on low speed for 1 minute. Scrape down the sides of the bowl with a rubber spatula. Beat for 2 minutes on medium speed or until well combined, scraping the sides down again if needed. Pour batter into prepared pan, smoothing the top with the rubber spatula.

Bake the cake until it springs back when lightly pressed with your finger, 35 to 38 minutes. Let cool on a wire rack for 20 minutes.

Meanwhile, prepare the frosting: Combine sugar, butter and milk in a small saucepan. Bring to a boil over medium-low heat, stirring constantly, 3 to 4 minutes. Boil, stirring constantly, for 1 minute. Remove from the heat, add chocolate chips, and stir until they are melted.

Let cool slightly but not completely, 3 to 4 minutes. Fold in peanuts and marshmallows until they are well combined and the heat from the frosting begins to melt the marshmallows.

Scrape the frosting onto the cooled cake, then spread evenly with a spatula until the frosting covers the entire cake. Let the cake rest 20 minutes before slicing.

(Store cake, covered with aluminum foil, at room temperature for 3 days or in the refrigerator for up to 1 week. Or freeze cake, wrapped in foil, for up to 6 months. Thaw the cake overnight in the refrigerator before serving.)