How to Make It
Heat oven to 350º F.
Make the crumb topping: In a food processor, pulse the pecans, brown sugar, flour, and butter until crumbly; transfer to a bowl and set aside. Wipe out the food processor.
Make the cake: Butter an 8-inch square baking pan. In the food processor, pulse the butter, flour, granulated sugar, baking powder, salt, and cinnamon until crumbly. In a large bowl, whisk together the eggs, milk, and vanilla. Add the flour mixture and mix just until combined.
Transfer the cake batter to the prepared pan. Sprinkle with the blackberries and crumb topping. Bake until a toothpick inserted in the center comes out clean, 55 to 60 minutes. Let cool completely in the pan before serving.