Blueberries or raspberries must be fresh or frozen - not thawed. May use a mixture of the two.
Preheat oven to 350F. Spray an 8" square baking pan with nonstick cooking spray. In a small bowl, whisk flour, sugar, baking soda and salt.Whisk in buttermilk and vanilla. Spread evenly into prepared pan.
Distribute berries evenly over batter. In a small bowl, combine sugar, cornstarch, and lemon peel. Sprinkle evenly over berries and bake in upper third of oven 35-40 minutes, or until golden brown and beginning to pull away form the edges of pan. Cool completely before cutting.