Heat oven to 350F. Grease and flour three 8- or 9-inch round cake pans. In medium bowl, beat 1/2 cup butter and 1 1/4 cups granulated sugar with electric mixer on medium speed, scraping bowl occasionally, until blended. Beat in 2 eggs, one at a time. Beat in chocolate, 3/4 cup milk and 1 teaspoon vanilla. Beat in remaining chocolate cake ingredients on low speed, scraping bowl constantly. Divide batter evenly between 2 pans.
Bake 25 to 30 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pans to cooling rack. Cool completely, about 1 hour.
In medium bowl, beat 1/2 cup butter and 1/2 cup granulated sugar with electric mixer on medium speed, scraping bowl occasionally, until blended. Beat in 1 egg; beat 1 minute. Beat in bananas, 1/4 cup sour cream, 1/2 teaspoon vanilla and the banana extract. Beat in remaining banana cake ingredients on low speed, scraping bowl constantly.
Pour into remaining pan.
Bake 23 to 25 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pan to cooling rack. Cool completely, about 1 hour.
In medium bowl, beat 1 cup butter with electric mixer on medium speed until fluffy. Beat in peanut butter, 1/2 cup of the powdered sugar, 1/4 cup milk and 1/2 teaspoon vanilla. Gradually beat in remaining 3 1/2 cups powdered sugar, beating after each addition until smooth.
Place 1 chocolate cake layer on serving plate; spread with 1/3 cup frosting.
Place banana cake layer on frosted layer; spread with 1/3 cup frosting. Top with chocolate cake layer. Frost side and top of cake with remaining frosting.
Garnish with peanuts and chocolate chips. Store in refrigerator.