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, beat the unsalted butter and granulated 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 pitcher 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.
07 - Gently fold the chopped basil and mixed berries into the batter. Be careful not to 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 to 35 minutes, or until a toothpick inserted in the center comes out clean.
09 - Let the cake cool 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 has cooled completely, spread the basil berry frosting evenly over the top and sides. Garnish with fresh berries and small basil leaves.