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 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 thoroughly after each addition. Stir in the vanilla extract until evenly distributed.
05 - In a small pitcher or measuring cup, whisk together the buttermilk and whole milk.
06 - Gradually add the dry ingredient mixture to the creamed butter, 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 chopped basil and mixed berries into the batter using a spatula. Take care not to overmix.
08 - Pour the batter into the prepared pan and level 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.
10 - Beat the butter until smooth and creamy. Gradually add the sifted powdered sugar and milk, beating until the frosting is light and fluffy. Fold in the chopped basil and mashed berries until fully incorporated.
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.