Preheat the oven to 325°F (160°C). Spray a 9x13 dish with non-stick spray and set aside.
Sift the flour into a mixing bowl and add the brown sugar, baking soda, salt, cinnamon and nutmeg and mix together.
Prepare your vegan buttermilk by adding 1 Tbsp fresh lemon juice to a measuring jug and then adding soy milk up to the 1 cup (240ml) line. Leave it for a minute to allow it to curdle.
Add the vegan buttermilk, oil, vanilla, apple cider vinegar and applesauce to the mixing bowl and mix in properly. The batter will be thick.
Add in the chopped apple pieces and fold in.
Transfer to your prepared baking dish and smooth down.
Prepare the cinnamon sugar topping by adding the sugar and cinnamon to a bowl and mixing together. Then add in softened vegan butter and mix it in until properly combined and the mixture is crumbly.
Sprinkle over the top of the cake.
Bake for 45 minutes or until a toothpick inserted into the center of the cake comes out clean.
Serve warm with vegan whipped cream or vegan ice cream.