01 - Preheat the oven to 350°F. Grease a 9-inch round cake pan and line the bottom with parchment paper.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, beat the unsalted butter and granulated sugar with an electric mixer on medium speed until pale, light, and fluffy, approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating thoroughly after each addition. Blend in the vanilla extract until evenly distributed.
05 - In a small pitcher or liquid measuring cup, combine the buttermilk and whole milk.
06 - Gradually add the dry ingredient mixture to the wet ingredients, alternating with the milk mixture. Begin and end with the dry ingredients. Mix just until combined to avoid overworking the batter.
07 - Gently fold the finely chopped basil and mixed berries into the batter using a spatula. Fold with care to avoid crushing the berries and overmixing.
08 - Pour the batter into the prepared cake pan and smooth the top with a spatula. Bake for 30 to 35 minutes, or until a toothpick inserted into the center emerges clean.
09 - Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before frosting.
10 - Beat the unsalted butter until smooth and creamy. Gradually add the sifted powdered sugar and whole milk, beating continuously until the frosting is light and fluffy. Fold in the chopped basil and mashed berries until evenly incorporated.
11 - Once the cake has cooled completely, spread the basil berry frosting evenly over the top and sides of the cake using an offset spatula.
12 - Arrange fresh berries and small basil leaves on top of the frosted cake for a vibrant finishing touch. Slice and serve.