Preheat oven to 375°F Generously grease 12 muffin cups or line them with paper cups.
Measure flour, sugar, cocoa, baking powder and salt into a large bowl. Stir with a fork until well mixed. In a medium-size bowl, whisk egg with milk, melted butter and vanilla. Make a well in center of flour mixture. Add milk mixture and stir just until combined. Do not overmix. Gently fold in berries, taking care not to break them up.
Spoon batter into muffin cups. Bake in center of preheated oven for 20 to 25 minutes or until a cake tester inserted in the center of a muffin comes out clean.
Cool muffins in tins on a wire rack for 5 minutes. Remove muffins from tins and continue to cool on rack. Serve warm, or cool completely and store in an airtight container in the refrigerator.