01 - Preheat oven to 350°F. 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 together using an electric mixer on medium-high speed until the mixture is pale, light, and fluffy, approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating well after each addition until fully incorporated before adding the next egg.
05 - Stir in the finely chopped fresh basil and vanilla extract until evenly distributed throughout the batter.
06 - Add the 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. Mix on low speed until just combined, being careful not to overmix.
07 - Gently fold the mixed berries into the batter using a spatula, taking care not to crush the fruit or overwork the batter.
08 - Pour the batter into the prepared cake pan and smooth the top with a spatula. Bake at 350°F for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean and the top is golden.
09 - Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely. If desired, whip the heavy cream with powdered sugar until soft peaks form and dollop over slices. Garnish with fresh berries and basil leaves.