Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease well.
Combine flour, cornmeal, ground flax, baking powder, baking soda and salt together in a large bowl.
In a medium bowl whisk egg with sugar until well combined. Whisk in buttermilk, oil and vanilla. Add egg mixture to flour mixture and stir until just barely combined. Add blueberries and raspberries and fold in just a few times. Immediately spoon into prepared muffin tin. Press a blackberry into the top of each muffin.
Bake 24 to 27 minutes or until a cake tester inserted into a muffin comes out clean. Cool in pan 5 minutes then remove and cool on a rack. Keep in an airtight container at room temperature for 2 days or in a resealable plastic bag in the freezer for up to 3 weeks.
Nutrients per serving: 221 calories, 4 g protein, 11 g fat (1 g saturated fat), 28 g carbohydrates, 2 g fibre, 18 mg cholesterol, 85 mg sodium
Best Health tip: To prevent the berries from bleeding into the batter, use them straight out of the freezer.