Preheat the Oven and Prepare the Pan - Preheat the oven to 350 degrees Fahrenheit. Spray the 13 x 9-inch cake pan with cooking spray.
Make the Cake Batter - In a large mixing bowl, use an electric mixer to beat together the cake mix, vegetable oil, and eggs. Mix on medium speed until fully combined. Alternatively, mix thoroughly by hand for two minutes.
Bake the Cake - Bake the cake for 30-35 minutes or until a toothpick inserted into the middle comes out clean. If there is batter or crumbs on the cake, continue to bake. Cool the cake for 10 minutes in the pan before transferring it to a wire rack to cool.
Combine the Peanut Butter and Canola Oil - In a small bowl, mix together the peanut butter and canola oil. Spread the peanut butter mixture on the cake while it is still warm.
Microwave the Frosting - In a microwave-safe bowl, melt the frosting for 25-30 seconds or until the frosting is pourable. Stir until the frosting is smooth.
Step Six: Decorate the Cake - Pour the heated frosting onto the cake, spreading it smoothly on. Let the cake stand until the frosting is set. Slice and serve the cake once set.