Preheat your oven to 350°F/175°C. Grease and line a 4.5x8.5 inch loaf pan.
Whisk all of the crumb topping ingredients together in a large jug, making sure it is evenly moist and there's no dry powder left. Place in the fridge.
Whisk the flour, cinnamon, baking soda, baking powder and salt together in a large bowl and set aside.
In a stand mixer fitted with the paddle attachment, whip up the butter and sugar together for 2 minutes at medium-high speed. You want it to look airy and fluffy.
Add the eggs and beat on medium for a minute. Make sure to stop the mixer and use a rubber spatula to scrape the sides down.
Add in the sour cream, shredded apples and vanilla extract and beat on medium speed for 1 minute. It will look slightly curdled which is fine!
Add in all of the dry ingredients and mix on low speed until completely combined. Then, slowly pour in your milk.
In a small bowl, whisk together the cinnamon and sugar.
Pour a third of the batter into your loaf pan, then sprinkle over half of your cinnamon sugar. Pour another third of the batter and the remaining cinnamon sugar. Finally, pour the rest of the batter on top. Use a small offset spatula to cover the cinnamon sugar underneath.
Sprinkle over your crumb topping. Bake for 65-75 minutes, check in after 30 minutes to make sure the topping isn't burning. If it's getting too brown, cover with foil. You'll know the loaf is done when you stick a toothpick in and it comes out with no crumbs. Allow the loaf to cool in the pan.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method. Just click on ‘Metric’ above the ingredients. Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Sour Cream: you can substitute full fat yogurt or applesauce for the sour cream. Storage: keep at room temperature for up to 2 days or in the fridge for 5 days. Freeze slices for up to 3 months and defrost overnight in the fridge.