Preheat the oven to 350°F (180°C). Grease and flour a 9×13 inch cake pan, or line the bottom of layer cake tins with parchment paper and grease the sides.

In a medium bowl, sift together the cake flour, baking powder, baking soda, and salt. Whisk to combine and set aside.

Combine the buttermilk and freshly squeezed lemon juice in a separate bowl, whisking them together to create a slightly thickened mixture. Set aside.

In a large bowl, beat the softened butter, granulated sugar, and lemon zest until light and fluffy, about 2-3 minutes using an electric mixer.

Beat in the vanilla extract. Add the eggs one at a time, beating well after each addition, scraping down the bowl sides after each egg.

With the mixer on low, add about one-third of the flour mixture, then half of the buttermilk-lemon juice mixture. Turn off the mixer and scrape the bowl. Repeat with another third of the flour and the rest of the buttermilk mixture. Finally, beat in the last third of the flour mixture.

Toss the blueberries with 2 teaspoons of flour to prevent sinking. Gently fold them into the batter with a spatula or rubber spoon.

Pour the batter into the prepared pan(s). Bake for 30-35 minutes for a 9×13 inch pan or 25-30 minutes for layer cakes, until the top is set and a toothpick inserted comes out clean.

Let the cakes cool in the pans. For layer cakes, avoid inverting while still hot to prevent breakage.

Beat the butter until soft, then add cream cheese and mix until smooth. Add lemon juice, then gradually beat in powdered sugar, adjusting frosting thickness with whipping cream as needed.

For a 9×13 inch cake, frost the cooled cake and optionally garnish with blueberries and lemon slices. For layer cakes, level domed tops if necessary, frost between layers, and crumb coat the entire cake. Chill for 20-30 minutes, then finish frosting the top and sides with swirls. Optionally decorate with blueberries.

Slice with a thin, sharp knife to serve.