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 unsalted butter and granulated sugar with an electric mixer on medium speed until pale, light, and fluffy, approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating thoroughly after each addition until fully incorporated. Stir in the vanilla extract.
05 - In a small pitcher or measuring cup, whisk together 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; avoid overmixing.
07 - Using a spatula, gently fold the chopped basil and mixed berries into the batter with minimal strokes to preserve the airy texture.
08 - Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 30 to 35 minutes, or until a toothpick inserted into the center emerges clean.
09 - Allow the cake to cool in the pan for 10 minutes, then invert onto a wire rack to cool completely before frosting.
10 - Beat the unsalted butter until smooth and creamy. Gradually add the sifted powdered sugar and whole milk, beating 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 using an offset spatula.
12 - Arrange fresh berries and small basil leaves on top of the frosted cake for a vibrant finishing touch.