If you don't have buttermilk on hand you can make sour milk. Add 2 teaspoons of lemon juice or plain white vinegar to your measuring cup, then pour the milk in to reach 2/3 cup. Let it sit briefly. This will make sour milk.
If you want to make this in a 9x13 pan you can add an additional 1/2 cup flour and 1/3 cup milk to the recipe. Double the crumb recipe and bake for 40 minutes.
Banana Bread Crumb Cake https://therecipecritic.com/banana-bread-crumb-cake/