Preheat the oven to 350°F. Grease and flour a 9x13-inch baking pan.

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

In a small bowl or measuring cup, stir together the buttermilk and fresh lemon juice. Let it sit for a few minutes; it will thicken slightly.

In a large bowl, beat the softened butter, granulated sugar, and lemon zest together with an electric mixer on medium-high speed until light and fluffy, about 2 to 3 minutes.

Add the vanilla extract and beat in the eggs one at a time, mixing well after each addition. Scrape down the sides and bottom of the bowl as needed.

With the mixer on low speed, add about one-third of the flour mixture to the butter mixture, mixing until just combined. Then add half of the buttermilk mixture, mixing until combined. Scrape down the bowl. Repeat with another one-third of the flour mixture, then the remaining buttermilk mixture, and finally the last one-third of the flour mixture. Do not overmix.

In a small bowl, toss the fresh blueberries with the 2 teaspoons of cake flour until evenly coated. Gently fold the blueberries into the batter with a rubber spatula.

Pour the batter into the prepared pan and spread it evenly. Bake for 30 to 35 minutes, or until the top is golden and a toothpick inserted in the center comes out clean. The cake should feel firm and springy when lightly pressed.

Let the cake cool completely in the pan on a wire rack before frosting.

In a large bowl, beat the softened butter with an electric mixer until creamy. Add the cream cheese and beat until smooth and well combined.

Mix in the lemon juice. Gradually add the powdered sugar, starting on low speed to avoid a cloud, then increasing to medium-high. Beat until the frosting is light and fluffy. If it seems too thick, add the heavy cream and beat until smooth.

Spread the frosting evenly over the cooled cake. Optionally, decorate with extra fresh blueberries and thin lemon slices.

Slice the cake into 15 pieces using a thin, sharp knife. Serve at room temperature or chilled.