Preheat oven to 200°C. Line a 20 cm pan with baking paper.
In a medium saucepan, combine the butter, cocoa powder and vegetable oil over medium-high heat until it just comes to the boil. Remove from the heat and set aside to cool down slightly.
Next, combine the sugars, eggs, buttermilk and vanilla in a large mixing bow
Slowly whisk in the melted butter mixture.
Add in flour, baking soda and salt, and stir just until combined. Fold in the white chocolate chips.
Pour into lined tin and bake for 20 minutes. Set aside to cool down.