Bring cream and sugar to simmer in saucepan, whisking to dissolve sugar. Remove from heat. Add chocolate; let stand 1 minute. Whisk until smooth. Whisk in peanut butter. Chill uncovered overnight.
Preheat oven to 350°F Butter three 9-inch-diameter cake pans with 1 1/2-inch-high sides. Line bottoms with parchment paper. Sift first 4 ingredients into medium bowl. Using electric mixer, beat butter and peanut butter in large bowl until blended. Beat in sugar. Beat in eggs, 1 at a time, then vanilla. At low speed, beat in flour mixture in 4 additions alternately with buttermilk in 3 additions.
Divide batter among pans and spread evenly. Bake cakes until tester inserted into center comes out clean, about 25 minutes. Cool cakes 5 minutes. Turn out onto racks; peel off parchment. Cool cakes completely.
Using electric mixer, beat cream cheese, 1 1/4 cups powdered sugar, butter, and vanilla in large bowl to blend. Whisk whipping cream and 3/4 cup powdered sugar in bowl until mixture holds medium-firm peaks. Fold into cream cheese mixture in 3 additions; chill until firm but spreadable, about 1 hour.
Place 1 cake layer, bottom side up, on 9-inch tart pan bottom. Spread with half of filling. Place another layer, bottom side up, on work surface. Spread with remaining filling; place atop first layer. Top with remaining cake layer, bottom side up.
Spread frosting over top and sides of cake. (Can be made 1 day ahead. Cover with cake dome; chill. Let stand at room temperature 2 hours before continuing.) Press candy and peanuts onto top of cake.