Cook apples with brown sugar, cinnamon and water until soft. Add cornstarch mixed with a little water and cook 1 minute more. Cool.
Mix melted butter, brown sugar and flour until crumbly.
Whisk melted butter + sugar. Add eggs and beat 2–3 minutes until fluffy. Mix in buttermilk and sour cream.
Combine flour, baking powder, baking soda and salt. Mix into the wet ingredients just until combined. Rest 10–15 minutes.
Add batter to a lined pan. Top with apples + crumble.
Bake at 175°C / 350°F for 50–55 min. Cover with foil after ~30 minutes if needed.
Cool slightly, slice, add vanilla ice cream and enjoy.
