01 - Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan, ensuring even coverage to prevent sticking.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until uniformly blended. 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 thoroughly after each addition until fully incorporated before adding the next. This ensures a smooth, emulsified batter.
05 - Stir in the finely chopped fresh basil leaves and vanilla extract, mixing until evenly distributed throughout the batter.
06 - Gradually add the dry ingredient mixture to the wet batter in three additions, alternating with the milk in two additions. Begin and end with the flour mixture. Mix on low speed until just combined — 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 airiness in the batter.
08 - Pour the batter into the prepared cake pan, smoothing the top with a spatula. Tap the pan gently on the counter to release any trapped air bubbles. Bake at 350°F (175°C) for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
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, spread over the cooled cake, and garnish with fresh berries and basil leaves.