To make filling:
If desired, whisk peanut butter into Majestic Milk Chocolate Frosting. Chill uncovered overnight.
To make cake:
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.
To assemble the cake:
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 Velvety Vanilla Frosting over top and sides of cake. (
DO AHEAD: 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.