Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease a 23 cm/9 in square cake tin and line it with baking parchment, making sure that you leave some overhanging on either side so it is easier to lift the baked cake out of the tin.
Prepare the crumble topping - mix the flour, sugar and cinnamon together in a bowl. Add the cold, diced vegan butter and rub it in using your fingertips until the mixture resembles breadcrumbs, is clumping together and no lumps of butter remain. Stir in the chopped nuts then pop the bowl in the fridge while you prepare the cake batter.
Place the flour, baking powder, bicarbonate of soda, cinnamon, nutmeg and salt in a large bowl and whisk to combine.
Mash the bananas well with a fork then add them to the bowl of flour along with the sugar, oil, milk, vinegar and vanilla extract.
Whisk until no dry lumps remain but be careful not to over-beat the batter as that can make the cake tough.
Pour the batter into the prepared tin and spread it level. Sprinkle the crumble topping evenly over the top.
Bake the cake for 40-45 minutes until it is firm to the touch and a skewer inserted into the centre comes out clean.
Leave to cool in the tin for 20 minutes then use the overhanging baking parchment to carefully lift it out onto a wire rack and leave to cool completely before slicing.