1 (15¼-oz.) pkg. devil’s food cake mix (plus ingredients listed on box for preparing cake)
Baking spray with flour
1 cup butter, softened
1 cup creamy peanut butter
3 cups unsifted powdered sugar
1 teaspoon vanilla extract
3/4 cup heavy whipping cream, divided
1 cup semisweet chocolate chips
Chopped roasted salted peanuts
Preheat oven to 350°F. Prepare devil’s food cake mix according to package directions. Divide batter evenly between 2 (9-inch) round cake pans coated with baking spray.
Bake in preheated oven until a wooden pick inserted in centers of cakes comes out clean, about 22 minutes. Cool in pans 10 minutes. Invert cakes onto wire racks; cool completely, about 45 minutes.
Beat softened butter and peanut butter with a stand mixer fitted with a paddle attachment on medium speed until smooth, about 2 minutes. Reduce mixer speed to low; gradually add powdered sugar, vanilla extract, and ¼cup of the cream. Beat on medium-high speed until fluffy, about 3 minutes. (You will have about 4 cups frosting.)
Spread 1½ cups frosting in a smooth layer over top of 1 cake layer using a large offset spatula. Top with remaining cake layer, and spread 1½ cups frosting in a smooth layer over top, spreading some of the frosting on top layer in a thin layer around sides of whole cake. Chill until frosting has firmed and set, about 30 minutes. Spread remaining frosting (about 1 cup) over sides of whole cake, smoothing as you go.
Microwave remaining ½ cup cream in a small microwavable bowl on HIGH until steaming, about 1 minute. Stir in chocolate chips until mixture is smooth. Cool 15 minutes.
Slowly pour about half of the chocolate mixture over the top of the cake. Spoon more of the mixture around top edge, allowing it to drip down sides, and then spoon any remaining chocolate mixture onto center of cake. If needed, use the spoon to nudge the icing down the sides of the cake. Smooth the top with an offset spatula, and sprinkle with peanuts.