In a bowl sift in the flour, baking powder, bicarbonate of soda and cocoa. Mix.
In a large bowl crack in the egg and whisk it. Add the 130g of sugar, oil and buttermilk; mix well.
Combine the 2 bowls together while mixing.
Filling: In a bowl carefully mix the cream cheese, caster sugar and vanilla.
Grease a muffin tray or line with muffin cases.
Fill each muffin case just over half then add the filling; a big teaspoon for every muffin. Put the rest of the mixture on top so the filling is completely surrounded.
Bake in the middle of the oven for 20-25 minutes. Afterwards if you're not using baking cases, let the muffins cool for 5-10 minutes in the moulds before removing.
Buttermilk! I had this trouble too, if you stock up and prepare you should be able to find it in your local supermarket. But if you work on a whim like I tend to: For each 250ml of fresh milk, add 1 tablespoon of lemon juice or white vinegar.