1Heat oven to 350°F. Grease bottom and sides of 13x9-inch pan, two 9-inch round cake pans or three 8-inch round cake pans; lightly flour. In large bowl, beat flour, granulated sugar, 1/2 cup peanut butter, 1 1/4 cups milk, the baking powder, salt, vanilla and eggs with electric mixer on low speed 30 seconds, scraping bowl constantly. Beat on high speed 3 minutes, scraping bowl occasionally.
2Pour two-thirds of batter (about 3 cups) into pan(s). Stir cocoa and baking soda into remaining batter. Drop chocolate batter by generous tablespoonfuls randomly in mounds onto peanut butter batter. Pull knife through batters in S-shaped curves in continuous motion for marbled design. Turn pan one-fourth turn; repeat marbling.
3Bake 13x9-inch pan 40 to 45 minutes, 9-inch rounds 30 to 35 minutes, 8-inch rounds 20 to 25 minutes, or until toothpick inserted in center comes out clean or until cake springs back when touched lightly in center. Cool 13x9-inch cake in pan on cooling rack. Cool rounds 10 minutes; remove from pans to cooling rack. Cool completely, about 1 hour.
4In medium bowl, mix powdered sugar and butter in medium bowl. Stir in vanilla, 1/4 cup milk and 1/4 cup peanut butter; beat until smooth and spreadable. Spread frosting over 13x9-inch cake or fill and frost cake rounds. Drop hot fudge topping by teaspoonfuls randomly over top; pull knife through frosting in S-shaped curves in continuous motion for marbled design.