1) Preheat oven to 325 degrees
2) Butter an 8-inch-square baking pan.
3) For filling, slice blackberries in half and toss with sugar and cornstarch. Set aside.
4) To make crumbs, in a large bowl, whisk together sugars, spices, salt and butter until smooth. Stir in flour with a spatula. It will be a solid dough.
5) To prepare cake, in a small bowl, stir together the sour cream, egg, egg yolk and vanilla.
6) In a separate mixing bowl, stir together flour, sugar, baking soda, baking powder and salt. Add butter and a spoonful of sour cream mixture and mix on medium speed with a hand mixer or stand mixer until flour is moistened. Increase speed and beat for 30 seconds. Add remaining sour cream mixture in two batches, beating for 20 seconds after each addition, and scraping down the sides of bowl with a spatula.
7) Scoop out about 1/2 cup batter and set aside.
8) Scrape remaining batter into prepared pan. Spoon blackberries over batter. Dollop set-aside batter over blackberries; no need for it to be even.
9) Using your fingers, break topping mixture into big crumbs, about 1/2 inch to 3/4 inch in size. They do not have to be uniform, but make sure most are around that size. Sprinkle over cake.
10) Bake cake until a toothpick inserted into center comes out clean of batter (it might be moist from blackberries), about an hour to an hour and 15 minutes. Cool before serving.