Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Place a 10-12 cup bundt tin in the fridge for 15 minutes then thoroughly brush it with melted vegan butter making sure that you get right into all of the nooks and crannies.
Mash the banana really well in a large bowl. Whisk in the sugar, milk, oil, vinegar and vanilla extract.
Stir in the flour, baking powder, bicarbonate of soda, cinnamon, nutmeg and salt until no dry lumps remain then fold in the chocolate chips, reserving a few to scatter over the top.
Transfer the batter to the prepared tin and spread it level. Bake for about 50-60 minutes until a skewer inserted into the centre comes out clean.
Leave the cake to cool in it's tin on a wire rack for 30 minutes before carefully flipping it out onto a wire rack and leaving it to cool completely.
Once the cake is cool, place the chopped chocolate and the coconut oil in a heatproof bowl set over a pan of gently simmering water. Do not let the base of the bowl touch the water.
Stir until the chocolate is just melted then remove it from the heat. Do not let it get too hot or it may sieze.
Allow it to cool and thicken slightly then pour the chocolate over the top of the cake and serve.