01 - Preheat oven to 350°F (175°C). Generously grease a 9-inch round cake pan with butter and dust with flour, tapping out any excess.
02 - In a medium mixing 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 until fully incorporated and the mixture is smooth.
05 - Stir in the finely chopped fresh basil and vanilla extract, mixing until evenly distributed throughout the batter.
06 - With the mixer on low speed, add the dry ingredients to the wet mixture in three additions, alternating with the milk in two additions. Begin and end with the flour mixture. Mix until just combined, being careful not to overmix.
07 - Using a spatula, gently fold the mixed berries into the batter with minimal strokes to avoid crushing the fruit and to keep the batter light.
08 - Pour the batter into the prepared cake pan, smoothing the top with a spatula. Bake 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 transfer to a wire rack to cool completely. If desired, whip the heavy cream with powdered sugar until soft peaks form, dollop onto slices, and garnish with fresh berries and basil leaves.