Crumb Topping

1. Preheat the oven to 325F/163C and line a 9-inch square pan with parchment paper and grease with non-stick spray. In a medium bowl whisk together the flour, granulated sugar, brown sugar, and cinnamon. Pour in melted butter and use a spatula to mix it all together to moisten everything, this will take a couple of minutes, you want a thick sand texture, thick clumps are good. Set aside in the freezer while you prepare the cake.

Apple Cake

1. In a large bowl whisk together the flour, baking soda, baking powder, and salt.
2. In the bowl of an electric stand mixer fitted with a paddle attachment or a large bowl with a handheld electric whisk beat the sugar, oil, and butter together for 2-3 minutes, scrape down the bowl after the first minute then continue mixing. You want a light fluffy mixture, don't cut corners here!
3. Add the eggs, one at a time, scraping down the bowl after each addition and making sure the egg is fully combined, there should be no trace of egg yolk. Add in the last egg and mix for at least two minutes.
4. In a separate bowl or measuring jug mix together the sour cream and vanilla extract. 
5. Gently fold in ⅓ of the dry ingredients until just almost combined, then gently fold in ½ of the sour cream until just combined, then repeat with ⅓ of the dry, the rest of the sour cream, and finish with the remaining flour. Just before the last bit of flour is fully mixed in add the chopped apples and mix to just combine, do not over-mix. Pour into the prepared pan and spread into an even layer.
6. Sprinkle all of the crumble on top of the cake batter into an even layer. Bake for 50-60 minutes, since the cake is covered in a crumble it is best to check by using a toothpick test, insert a toothpick into the center of the cake, moist crumbs are okay but if there is any wet batter the cake will need to cook longer.
7. Let the cake cool on a wire rack for at least 30 minutes before using the parchment overhang to gently and quickly remove it from the pan. Best served the day it is made but leftovers will last in an airtight container for up to 5 days.