Ingredients

• Nonstick vegetable oil spray
• 1 1/4 cups all-purpose flour
• 1 cup sugar
• 3/4 cup natural unsweetened cocoa powder
• 1 1/2 teaspoons kosher salt
• 1 teaspoon baking soda
• 1/4 cup vegetable oil
• 1 teaspoon vanilla extract
• 1/2 cup finely chopped semisweet or bittersweet chocolate (3 1/2 ounces)
• 1/2 cup sugar
• 1/4 cup egg whites (from about 2 large eggs)
• 1 1/2 teaspoon vanilla extract
• 1/4 cup creamy peanut butter
• 6 tablespoons (3/4 stick) chilled unsalted butter, cut into 1/4" cubes
• Kosher salt
• 1/4 cup chopped semisweet or bittersweet chocolate (about 1 3/4 ounces)
• 1/4 cup chopped unsalted, dry roasted pinenuts
• Special equipment: An 8x8x2" cake pan

Instructions

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: 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. 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. Cut into 2" squares.
3. 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.
4. 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.
5. Cut into 2" squares.