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 room-temperature butter and granulated sugar with an electric mixer on medium speed until light and fluffy, approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract until fully incorporated.
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 gently just until combined; do not overmix.
07 - Gently fold the chopped fresh basil and mixed berries into the batter using a spatula, taking care not to overmix.
08 - Pour the batter into the prepared cake pan and smooth the top level. 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 it to a wire rack to cool completely before frosting.
10 - Beat the room-temperature butter until smooth and creamy. Gradually add the sifted powdered sugar and milk, beating continuously until the frosting is light and fluffy. Fold in the 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 - Arrange fresh berries and small basil leaves on top of the frosted cake for garnish. Slice and serve.