Preheat the oven to 180℃. Grease and line with parchment paper your Square size 10 baking tin. Sieve the flour to remove the rumps.

Beat butter and sugar until fluffy and light, add in the eggs one at a time. Add the oil and essence and continue mixing. Fold in the flour alternating with the buttermilk. Don't over mix. Divide the batter into two portions. Add cocoa into one and mix.

Pour in the baking tin alternating btwn the batters. Bake in the middle rack for btwn 40-50 minutes