Preheat oven to 375°F Coat a 9-inch square baking pan with cooking spray.
To prepare crumb topping, place flour, brown sugar, sugar and cinnamon in a food processor; pulse to combine. Add butter and process until crumbly. Add walnuts and pulse 3 times or until evenly distributed.
To prepare the cake, sift flour, baking powder, baking soda and salt. Combine butter, sugar, egg and vanilla in a mixing bowl and beat until evenly blended. Turn to low speed; add flour mixture alternately with buttermilk and beat until blended.
Stir in 1 cup berries. Turn into pan and spread evenly. Scatter remaining 1 cup berries over top and sprinkle with crumb topping. Bake 45 minutes or until nicely browned and edges of cake begin to pull away from sides of pan. Cool on wire rack.