01 - Preheat the oven to 350°F. Grease and line a 9-inch round cake pan 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, cream the butter and sugar with an electric mixer until light and fluffy, about 3 to 4 minutes.
04 - Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until fully incorporated.
05 - In a small jug or measuring cup, combine 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 to avoid a dense crumb.
07 - Gently fold the chopped basil and mixed berries into the batter using a spatula. Do not 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 - Let the cake cool in the pan for 10 minutes, then turn it out onto a wire rack to cool completely before frosting.
10 - Beat the butter until smooth and creamy. Gradually add the powdered sugar and milk, beating until light and fluffy. Stir 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 using an offset spatula.
12 - Arrange fresh berries and small basil leaves on top of the frosted cake for a vibrant finish. Slice and serve.