Preheat your oven to 350°F (175°C) and line a muffin tray with cupcake liners.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs, one at a time, mixing well after each addition, then stir in the vanilla extract.
In another bowl, combine the flour, baking powder, and salt, mixing well.
Alternate adding the dry mixture with the milk to the butter mixture, beginning and ending with the flour mixture.
Gently fold in the mixed berries, being careful not to overmix.
Spoon the batter into the prepared cupcake liners, filling each about 2/3 full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted comes out clean.
Allow the cupcakes to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
While the cupcakes are cooling, whip the heavy cream with powdered sugar until soft peaks form.
Once the cupcakes are fully cooled, frost them with the whipped cream and top with additional berries if desired.