Preheat the oven to 400°F. Lightly grease and flour the wells of a muffin pan, or line the pan with paper liners, and grease the liners., In a medium-sized bowl, whisk together the flour, sugar, baking powder, salt, and cake enhancer., Whisk together the milk, yogurt, eggs, and flavor, and gently stir into the dry ingredients. Stir in the melted butter., Gently stir in the fruit., Spoon the batter into the prepared muffin cups, filling each nearly full. A level (to slightly heaped) muffin scoop of batter for each muffin works well here., Combine the topping ingredients, mixing until well combined and crumbly., Sprinkle a generous tablespoon of topping over each muffin., Bake the muffins for 16 to 18 minutes, until they've domed nicely and they're lightly browned., Remove the muffins from the oven, and transfer them to a rack to cool.