Frozen dark sweet cherries may be substituted for fresh sour cherries. Let frozen cherries thaw before using.
Preheat oven to 350 degrees. Butter a 9-inch square baking dish, and dust with flour, tapping out excess.
Make the topping: Stir together butter, flour, sugars, salt, and cinnamon.
Make the cakes: Whisk together flour, baking powder, and salt in a bowl. With a mixer, cream butter and granulated sugar in another bowl until pale and fluffy. Beat in eggs and vanilla. Working in alternating batches, add flour mixture and buttermilk, beginning and ending with flour mixture. Beat until just combined. Pour into prepared dish, and smooth with an offset spatula. Dot top with cherries, and sprinkle with crumb topping.
Bake until golden and a tester inserted into center comes out clean, about 1 hour. Let cool before cutting.