In a large bowl combine the sugar and flour, then add the diced butter and using your fingers crumble to form a breadcrumb like mixture leaving some bigger lumps of butter. Next add the cinnamon and walnuts and mix, then place the bowl in the fridge.
Grease and line a 1 litre loaf tin with parchment. In a bowl mash the ripe bananas and add the oil, set aside. Using an electric whisk beat together the butter and sugar until it is light and fluffy, then add the eggs one at a time, mixing between each. Next add the flour, milk, vanilla, spices and mashed banana and carefully fold together making sure you don’t overmix. Pour into the loaf tin and top with the streusel mix, then bake in the oven for 50 minutes until a skewer comes out clean.
Allow to cool slightly in the tin and then remove and cool on a wire rack before slicing and serving.