01 - Preheat 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 fully incorporated.
05 - In a small measuring cup or pitcher, combine the buttermilk and whole milk.
06 - Gradually add the dry ingredient mixture to the creamed butter mixture, 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 fresh basil and mixed berries into the batter using a spatula. Do not overmix.
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 comes out clean.
09 - Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
10 - Beat the unsalted butter until smooth and creamy. Gradually add the sifted powdered sugar and whole milk, beating continuously until light and fluffy. Fold in the finely chopped basil and mashed berries until evenly distributed.
11 - Once the cake has cooled completely, spread the basil berry frosting evenly over the top and sides of the cake.
12 - Garnish the frosted cake with fresh berries and small basil leaves. Slice and serve.