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 measuring cup or bowl, whisk together 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, mixing just until each addition is incorporated.
07 - Gently fold the chopped fresh basil and mixed berries into the batter using a spatula. Be careful not to overmix to avoid crushing the berries.
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 turn it out onto a wire rack to cool completely before frosting.
10 - Beat the unsalted butter with an electric mixer until smooth and creamy. Gradually add the sifted powdered sugar and whole milk, beating until the frosting is light and fluffy. Fold in the chopped basil and mashed berries until evenly blended.
11 - Once the cake has cooled completely, spread the basil berry frosting evenly over the top and sides of the cake.
12 - Arrange fresh berries and small basil leaves on top of the frosted cake for garnish. Slice and serve at room temperature.