You will need -
10oz plain flour
1tbsp baking powder
3.5oz caster sugar
2 medium free range eggs
9fl oz milk
6tbsp sunflower oil
1tsp vanilla extract
Mix the dry ingredients except the cocoa together.
Beat the wet ingredients and then stir into the dry ingredients.
Split the mixture between two bowls and mix the cocoa into one of them.
Put 12 cases in your muffin pan and spoon in the mixture alternating between the plain and chocolate mixture. The smaller the spoonfuls the more marbly the muffins will be.
Bake at 200C for about 20mins until the tops pop back up when gently pressed.