Preheat the oven to 350F degrees. Grease and flour a 9x13 inch cake pan.
In a medium bowl sift together the flour, baking powder and salt.
In a large bowl beat together the butter, sugar and lemon zest until fluffy. (About 2-3 minutes).
Add in the vanilla extract and beat in the eggs 1 at a time.
In a liquid measuring cup whisk together the lemon juice and buttermilk.
With the mixer on low speed, beat about 1/3 of the flour mixture into the butter mixture followed by about 1/3 of the milk mixture.
Toss the blueberries with the 2 teaspoons of flour, then gently fold into the batter with a spatula.
Pour the batter into the prepared pan. Bake for 25-30 minutes, or until an inserted toothpick comes out clean.
Beat the butter until soft, then mix in the cream cheese.
Add in the lemon juice and mix in 3 cups of the powdered sugar, starting with the mixer on low speed and then turning up to medium as it starts to combine.
Beat in 1 tablespoon of whipping cream, and then mix in the rest of the powdered sugar as necessary, going little by little and adding more cream if needed.
Frost the cooled cake, and optionally decorate with blueberries and lemon slices.
*If you don't have cake flour, measure out 2 1/4 cups all-purpose flour. Remove 1/3 cup and replace it with 1/3 cup cornstarch. Sift together 4 times.
**Use unsalted butter, softened to room temperature.