Pre-heat oven to 180°C/356°F. Line the bottom of a 10-inch springform pan with parchment paper. Grease the sides of the pan and the parchment paper with a little vegan butter.
Cinnamon crumble: In a medium bowl, combine all-purpose flour, granulated sugar, brown sugar, salt and ground cinnamon. Pour in the melted vegan butter and mix until well combined. Set aside.
Cinnamon apples: In a small bowl, toss the cubed apples with the ground cinnamon.
Cake batter: In a small bowl, combine the oat milk with apple cider vinegar. Set aside. In a large bowl, beat the granulated sugar with the butter for 2-3 minutes. Add in the vanilla extract and beat for a further 2-3 minutes. Pour in the homemade buttermilk and mix! It'll look clumpy, but don't worry! Add in the all-purpose flour, baking soda, baking powder, cornflour and salt. Fold together until well combined.
Final touches: Pour the cake batter into a lined baking pan. Top with the cinnamon apples. Try and keep the apples in a single layer, this way every bite will be packed with flavor. Add the crumble topping over the apples. Bake for 45-55 minutes or until baked. While it bakes, beat the softened vegan cream cheese with the icing sugar. Add in oat milk and whisk until you achieve the perfect drizzling consistency. Allow the cake to cool for 10 minutes in the pan. Pop open the spring form pan. Drizzle with a little vegan cream cheese icing.