Preheat your oven to 180c and lightly grease your cake tins. I've used two 20cm loose bottom cake tins here.
In a mixing bowl, combine the olive oil, peanut butter and sugar, then beat together until well combined. Whisk in your milk mixture.
Fold your dry ingredients into the batter and divide this between your two cake tins. Pop into the oven for 30-35 minutes to bake.
Once your cakes are cooked through, allow to cool on a wire wrack whilst you prepare the frosting.
Using an electric whisk, beat the butter until light and fluffy, then gradually incoorporate your icing sugar and finally, the peanut butter.
Spread a generous layer of the frosting onto one of the cakes and carefully sandwich the other on top.
Divide the remaining buttercream in half and beat the cocoa powder into one half. Spread the icing over the entire cake (chocolate on the bottom), then smooth over using a palette knife to create an ombré effect.