Preheat oven to 350F. Spray 9x9 square baking pan. Whisk flour, sugar, cocoa, baking soda, and salt in baking pan. Add oil, vinegar, and vanilla. Pour water into pan and mix until just a few streaks of flour remain. Using a rubber spatula, spread batter evenly in pan. 

Bake 23-25 minutes. Don’t overbake or the cake will be dry. Cool completely in the pan set on a wire rack.

Peanut Butter Buttercream Frosting Combine sugar and egg whites in medium bowl set over a saucepan of  simmering water. Whisk constantly until sugar dissolves, 3–4 minutes. Remove bowl from saucepan and with an electric mixer, beat on high until cool and thick, 5–6 minutes. Beat in vanilla, then peanut butter. With the mixer running, add butter a few pieces at a time, beating to blend between additions. Add salt. Frosting is more soft and creamy than a traditional Buttercream. If it seems too runny, refrigerate for an hour to firm. The finished texture is like cream whipped to soft peaks.

Run a thin knife around cake pan to release cake and invert onto a serving plate. Spread Frosting over top. Garnish with chopped chocolate and peanuts. The cake may be made and frosted a day ahead. Store airtight at room temperature.