• Preheat the oven fan 180°C
• Whisk together the butter and both types of sugar until fully combined
• Crack the eggs into the same bowl. Add the flour too. Whisk until smooth (20-30 seconds on high).
• Divide the mix into two bowls, with equal amounts in each. (in the pic, you can see: a mug with cocoa powder & water, a bowl with mashed banana and two silver bowls with half the cake mix in each)
  
• In one half of the mix, add the cocoa powder & water. Whisk until combined.
• In the other half, add the mashed banana. Whisk until combined.
• Line your muffin tray with cases/your loaf tin with baking paper.

If you're making a Loaf:

• Add a couple of spoonfuls of the chocolate mix to the base of your loaf tin. Add some spoonfuls of banana mix. Repeat until you’ve used all your mix. Draw one big "S" through the loaf to give it the perfect marble
  
• Bake for 20mins and then check on it – if the top is starting to brown, lay a piece of foil loosely over the top (this will stop the top cooking so quickly, but the middle will keep cooking!). If you’d like to add chocolate buttons/cacao nibs, do that now!
• Return to the oven for another 10-20mins, or until a knife comes out just clean (it’s quite a wet mix so don’t expect the knife to be totally clean. It’ll also keep cooking whilst its cooling even after its been removed from the oven!)

If you're making Muffins:

• Add a spoonful of the chocolate mix to each muffin case. Add a spoonful of the banana mix. Repeat until you’ve used all your mix and the muffin cases are ¾ full. 
• Draw an "S" into each muffin with a knife, to give the muffins the perfect marble.
• Bake for 20-25mins, or until a knife comes out just clean. If you’re adding chocolate buttons, place them on top after 15mins of cooking time.