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 all-purpose 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 3–4 minutes.
04 - Add 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 wet mixture, alternating with the milk mixture, beginning and ending with the dry ingredients. Mix just until combined to avoid overworking the batter.
07 - Gently fold in the finely chopped basil and mixed berries. Do not overmix to keep the berries intact.
08 - Pour the batter into the prepared cake pan and level the top with a spatula. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
09 - Cool the cake in the pan for 10 minutes, then transfer to a wire rack to cool completely before frosting.
10 - Beat the butter until smooth and creamy. Gradually add the sifted 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, frost the top and sides evenly with the basil berry frosting.
12 - Decorate the frosted cake with fresh berries and small basil leaves. Slice and serve.