Cook apples, sugar, cinnamon and water over medium heat until softened.
Mix cornstarch with a splash of water, stir it in and cook for another minute.
Cool completely.
For the crumble, mix melted butter, brown sugar and flour until crumbly.
Set aside.
For the cake, whisk melted butter and sugar.
Add the eggs and beat for 2–3 minutes until lighter and fluffy.
Mix in buttermilk and sour cream.
In a separate bowl, combine flour, baking powder, baking soda and salt.
Add the dry ingredients to the wet and mix just until combined.
Let the batter rest for 10–15 minutes.
Transfer to a lined loaf pan.
Top with the apple filling and crumble.
Bake at 175°C / 350°F for 50–55 min.
If the top gets too dark, loosely cover with foil after about 30 minutes.
Cool on a rack for 30–60 minutes.
Serve warm with vanilla ice cream
