01 - Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan with butter, then 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 thoroughly after each addition and scraping down the sides of the bowl as needed to ensure even incorporation.
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 additions, alternating with the whole milk in two additions. Begin and end with the flour mixture. Mix on low speed just until each addition is incorporated — do not 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 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 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 turn it out onto a wire rack to cool completely. If using the optional topping, whip the heavy cream with powdered sugar to soft peaks, dollop over the cooled cake, and garnish with fresh berries and basil leaves.