Chocolate-Peanut Butter Fun Cake

Preparation

1. For chocolate cake:

   Step 1

   Preheat oven to 350°F. Coat bottom and sides of pan with nonstick spray; line bottom of pan with parchment paper. Whisk flour and next 4 ingredients in a large bowl. Add oil, vanilla, and 1 1/4 cups water; whisk until smooth. Fold in chopped chocolate. Scrape into prepared pan; smooth top. Bake until a tester comes out clean when inserted into center, 35-40 minutes. Let cool completely in pan on a wire rack.

2. For peanut butter buttercream:

   Step 2

   Combine sugar and egg whites in a medium metal bowl set over a saucepan of simmering water. Whisk constantly until sugar dissolves and mixture is hot to the touch, 3-4 minutes. Remove from heat; using an electric mixer, beat on high speed until cool and thick, 5-6 minutes. Beat in vanilla, then peanut butter. With mixer running, add butter a few pieces at a time, beating to blend between additions. Season with salt.

   Step 3

   Run a thin knife around pan to release cake. Invert cake onto a serving plate. Spread peanut butter buttercream over top. Garnish with chopped chocolate and peanuts. DO AHEAD: Can be made 1 day ahead. Store airtight at room temperature.

   Step 4

   Cut into 2" squares.