Crumb Topping
1 cup packed dark brown sugar
1/2 cup granulated sugar
1/2 teaspoon salt
1 1/2 tablespoons cinnamon
1 cup (2 sticks) unsalted butter, melted (and still warm)
2 1/2 cups all-purpose flour

Cake
2 1/2 cups all-purpose flour
3/4 teaspoon baking powder
1 teaspoon baking soda
1/2 teaspoon salt
12 tablespoons (1 1/2 sticks) unsalted butter, at room temperature
1 1/2 cups sugar
2 large eggs, at room temperature
1 1/4 cups sour cream, at room temperature
1 teaspoon vanilla extract