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 flour, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, beat the butter and sugar with an electric mixer on medium speed until pale, light, and fluffy, about 3 to 4 minutes.
04 - Add eggs one at a time, beating well after each addition. Stir in the vanilla extract until fully incorporated.
05 - In a small measuring cup or bowl, stir together the buttermilk and whole milk.
06 - Gradually add the dry ingredients to the butter mixture, alternating with the milk mixture. Begin and end with the dry ingredients. Mix just until combined and no dry streaks remain.
07 - Gently fold the chopped basil and mixed berries into the batter using a spatula. Be careful not to overmix.
08 - Pour the batter into the prepared pan and smooth the top. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
09 - Let the cake cool in the pan for 10 minutes, then turn it out onto a wire rack to cool completely.
10 - Beat the butter until smooth and creamy. Gradually add the powdered sugar and milk, beating until light and fluffy. Fold in the chopped basil and mashed berries until evenly distributed.
11 - Once the cake is completely cool, 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. Slice and serve.