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 together using an electric mixer on medium speed until the mixture becomes 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. Scrape down the sides of the bowl as needed.
05 - Stir the finely chopped fresh basil and vanilla extract into the batter until evenly distributed throughout the mixture.
06 - Add the dry ingredient mixture to the wet batter in three 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 - Using a spatula, gently fold the mixed berries into the batter with careful, sweeping motions to avoid crushing the fruit and to maintain an even distribution.
08 - Pour the batter into the prepared cake pan, smoothing the top with a spatula. Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown.
09 - Remove the cake from the oven and allow it to cool in the pan for 10 minutes before inverting onto a wire rack to cool completely. If desired, whip the heavy cream with powdered sugar until soft peaks form and spread over the cooled cake. Garnish with fresh berries and basil leaves before serving.