Preheat your oven to 320°F (160°C). Grease or line two 6-inch cake tins.
Roughly chop Oreos into bite-sized pieces.
Mix dry ingredients (flour, baking powder, baking soda, cornflour, salt) in a bowl.
Cream together sugar, vegetable oil, and butter until light and fluffy.
Add eggs one at a time, mixing well after each addition. Stir in buttermilk, vinegar, and vanilla.
Fold in half of the dry mix, then the remaining buttermilk, and finally, the rest of the dry mix.
Gently fold in the chopped Oreos.
Pour the cake batter into the prepared tins. Bake them for about 35 minutes.
Cool the cake in the tins for 20 minutes, then transfer it to a wire rack to cool completely.
Make Oreo buttercream frosting.