01 - Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan with butter and dust lightly with flour, tapping out any excess.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until evenly distributed. Set aside.
03 - In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium-high speed until pale, light, and fluffy, approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating thoroughly after each addition and scraping down the sides of the bowl as needed to ensure a uniform mixture.
05 - Stir in the finely chopped fresh basil leaves and vanilla extract until evenly distributed throughout the batter.
06 - Add the reserved dry ingredient mixture to the wet batter in three separate additions, alternating with the whole milk in two additions. Begin and end with the flour mixture. Fold gently until just combined, taking care not to overmix.
07 - Gently fold the mixed berries into the batter using a spatula, distributing them evenly without crushing the fruit.
08 - Pour the batter into the prepared cake pan and smooth the top with a spatula. Bake on the center rack at 350°F (175°C) for 30 to 35 minutes, or until a wooden toothpick inserted into the center comes out clean and the top springs back when lightly pressed.
09 - Allow the cake to cool in the pan for 10 minutes, then invert onto a wire rack to cool completely. If desired, whip the heavy cream with powdered sugar to soft peaks and dollop over slices, garnishing with fresh berries and basil leaves.