Add butter to a skillet and cook over medium-low temperature until the butter browns. Cool. You will use a portion for both the cake and crumb topping.
Preheat oven to 350°F and grease an 9-inch springform pan with butter.
Prepare the crumble topping by adding 1 cup flour,1/2 cup brown sugar, 1/4 cup sugar, 1 tsp cinnamon, and 1/2 tsp salt to a bowl and combine. Add 1/3 cup of browned butter and mix it together with a fork and set it aside.
Peel and chop the apples into small pieces per your preference
In a stand mixer using the paddle attachment, cream the rest of the browned butter and ¾ cup brown sugar/4 cup of brown sugar until fluffy. Add 2 eggs and then mix in 1/2 cup sourdough discard.
Next add 1 1/2 cups flour,1 tsp baking powder, 1/2 tsp baking soda, 1 tsp cinnamon, and 1/4 tsp salt. Mix well and then pour in 1/4 cup of milk.
Fold in apples by hand.
Fill the greased springform pan with the cake batter and then top with the browned butter crumble.
Bake for an hour or until a toothpick comes out clean.
Cool for an hour or more before removing from the springform pan and serving