Banana Crumb Cake

Name: Banana Crumb Cake
Ingredients: For the crumble:; 1/4 cup unsalted butter; 1/3 cup brown sugar, packed; 1/4 teaspoon salt; 1/2 teaspoon vanilla extract; 1/4 teaspoon cinnamon; 1/2 cup plus 2 tablespoons al-purpose flour; For the cake:; 1/4 cup unsalted butter, at room temperature; 1/4 cup sugar; 1/4 cup brown sugar, packed; 1 large egg; 1/4 cup buttermilk; 1 teaspoon vanilla extract; 1 cup mashed banana; 1 cup all-purpose flour; 1/2 teaspoon baking soda; 1/2 teaspoon baking powder; pinch of salt