Preheat the oven to 350°F (175°C). Grease and flour three 9-inch round cake pans.

Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

Cream Butter and Sugar: In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy (about 3-4 minutes).

Add Eggs and Flavor: Add the eggs one at a time, mixing well after each addition. Stir in the lemon zest and vanilla extract.

Combine Wet and Dry: Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk. Begin and end with the dry ingredients.

Fold in Blueberries: Gently fold in the fresh blueberries with a spatula.

Bake Cakes: Divide the batter evenly among the prepared cake pans. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.

Cool: Allow the cakes to cool in the pans for 10 minutes, then transfer them to wire racks to cool completely.

Make Frosting: Beat the softened butter until creamy. Gradually add the powdered sugar, lemon juice, and lemon zest. Mix until smooth, adding milk as needed for desired consistency.

Assemble Cake: Once the cakes are completely cool, place one layer on a serving plate and spread a layer of frosting on top. Repeat with the second layer. Place the third layer on top and frost the top and sides of the cake.

Chill: Chill the cake for at least 30 minutes before slicing and serving to set the frosting.