Preheat the oven to 170°C. Lightly grease a 9x5 loaf tin and line with a strip of parchment paper.
In a large mixing bowl, use a whisk to combine the banana, sugar, vanilla extract, groundnut oil, and the almond milk. Stir until homogenous.
Sift the flour, baking powder, and salt directly into the banana batter and use a rubber spatula to lightly fold the batter together, trying not to overmix.
In a medium bowl, whisk together the cocoa powder with 3 tbsp of the boiling water. It should be thick but smooth. Add half of the banana batter to this bowl and whisk lightly to combine.
Add the remaining 3 tbsp of boiling water to the remaining un-chocolatey banana batter, and whisk together lightly.
Place alternate spoonfuls of the two batters into the prepared loaf tin, and then use a knife to swirl the two together.
Bake in the preheated oven for 45-50 minutes, or until a skewer inserted into the centre of the cake comes out clean. Leave to cool completely before slicing.