Preheat the oven to 375°F (190°C) and line muffin pans with liners or lightly grease with cooking spray. This recipe yields 16-18 muffins, so you'll need two pans.
Start by chopping the berries to your desired size. I left the blueberries as they were and cut the blackberries and raspberries into halves or thirds. Then I sliced and chopped the strawberries into smaller chunks. Rinse and drain well. Juice and zest your lemon. One large lemon should be enough, but buy a second just in case.
In a medium bowl, whisk together the flour, baking powder and salt.
In a large bowl, cream the butter, yogurt and sugar until smooth. Beat in the eggs one at a time until incorporated. Mix in the vanilla, lemon juice and zest.
Mix in half of the flour mixture on low speed, until just combined. Stir in the whole milk and then the remaining flour until just combined.
Set aside 1/2 cup of berries. Gently fold the remaining berries into the batter. Fill each prepared muffin cup 2/3 full. For more even baking, fill any empty cups 2/3 full of water.
Top the muffins with the 1/2 cup of berries that you set aside. Bake for 16-20 minutes, or until lightly browned and a toothpick inserted into the center of a muffin comes out clean. (I recommend baking one pan at a time.)
Let the muffins rest in the pan for 5 minutes before removing to a wire cooling rack. Store in an airtight container in the refrigerator or freezer. Enjoy!