Preheat your oven to 350°F (175°C) and grease two round cake pans thoroughly with butter or cooking spray to prevent sticking.
In a mixing bowl, whisk together all-purpose flour, baking powder, baking soda, and salt until well combined.
In another large bowl, mix vegetable oil and brown sugar until smooth before adding in peanut butter followed by eggs one at a time along with vanilla extract.
Gradually add milk to your wet mixture while continuing to mix until fully incorporated before folding in dry ingredients gently until just combined.
Divide batter evenly between prepared pans and bake for about 30 minutes or until a toothpick inserted comes out clean.
In a separate bowl, beat softened unsalted butter with cream cheese until creamy then gradually mix in peanut butter followed by powdered sugar and vanilla extract until desired consistency is achieved.