Preheat the oven to 350°F.
Gently wash the fresh strawberries, blueberries, raspberries, and blackberries, then place them on layers of paper towels to dry. Cut the tops off the strawberries and slice, then place on paper towels to absorb excess moisture.
Spray a 6-well jumbo muffin tin with Baker's Joy Nonstick Baking Spray. Evenly distribute each of the berries among the six muffin tin wells and bake for 10 minutes. While the berries are baking, prepare the cake batter. Start by placing the room-temperature unsalted butter and sugar in a large bowl. Then, using an electric hand mixer, beat them together for 2-3 minutes until light and fluffy.
Then, add the vanilla extract and eggs, and beat until well combined.
Add the all-purpose flour, baking powder, and salt, and beat for about a minute, or until thoroughly combined.
Next, add the milk and beat for about a minute, or until a smooth batter is formed.
After 10 minutes, remove the pan from the oven. Using an ice cream scoop, evenly distribute the batter among the 6 wells. Return the pan to the 350°F oven and bake for 20-23 minutes.
Once baking has been completed, remove the pan from the oven and allow it to cool for 10-15 minutes on a wire rack. Run a warm knife around the edges of the cakes to loosen them up and prevent sticking before inverting them.
Place a wire rack over the top of the pan, then carefully flip both together. Place the rack on a flat surface, gently tap the pan to release the cakes, and carefully lift the pan off. Allow the cakes to cool completely, then serve with fresh whipped cream or vanilla ice cream.
Yield 6 servings.