Preheat the oven and prepare the baking pan as per your chosen method.

In a mixing bowl, combine whole milk, sour cream, egg whites, fresh lemon juice, lemon zest, and vanilla extract.

In another bowl, cream softened unsalted butter and granulated sugar until light and fluffy.

Gradually add the flour, baking powder, and kosher salt to the cream mixture, alternating with the wet ingredients, mixing until well combined.

Gently fold in the fresh blueberries into the batter.

Pour the batter into the prepared baking pan and smooth the top.

Bake in the preheated oven for 35 minutes or until a toothpick inserted into the center comes out clean.

While the cake cools, prepare the blueberry topping by combining granulated sugar, water, and any remaining blueberries in a saucepan.

Cook over medium heat until the mixture thickens slightly, then set aside to cool.

For the frosting, beat softened unsalted butter with confectioners' sugar, lemon juice, and kosher salt until creamy and fluffy.

Once the cake has cooled, frost with the lemon frosting and top with the blueberry mixture.