. Prepare Oven and Pan(s):

• Preheat the oven to 350°F (180°C).  
• For 9×13 Pan: Grease and flour the pan thoroughly.
• For Layer Cake: Grease three 8-inch round cake pans. Line the bottoms with parchment paper rounds, then grease and flour the parchment and sides of the pans.

2. Combine Dry Ingredients:

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

3. Combine Buttermilk & Lemon Juice:

• In a liquid measuring cup or small bowl, combine the room temperature buttermilk and freshly squeezed lemon juice. Whisk with a fork and set aside. (It may look slightly curdled; this is normal).

4. Cream Butter, Sugar, and Zest:

• In a large bowl using an electric mixer (stand or handheld), beat the softened butter, granulated sugar, and lemon zest on medium-high speed until light and fluffy (about 2-3 minutes).

5. Add Eggs and Vanilla:

• Beat in the vanilla extract.
• Add the room temperature eggs one at a time, beating well after each addition until fully incorporated. Turn off the mixer and scrape down the sides and bottom of the bowl after each egg.

6. Alternate Flour and Buttermilk Mixtures:

• With the mixer on low speed, add about one-third of the flour mixture to the butter mixture. Beat until just barely combined.  
• Add about half of the buttermilk/lemon juice mixture. Beat on low until just combined.
• Repeat the process: add another third of the flour mixture, mix briefly. Add the remaining buttermilk/lemon juice mixture, mix briefly.
• Finally, add the last third of the flour mixture and beat on low only until just combined. Do not overmix. Scrape the bowl.

7. Fold in Blueberries:

• In a small bowl, gently toss the fresh (or frozen, unthawed) blueberries with the 2 teaspoons of cake flour (or AP flour) until lightly coated.
• Gently fold the flour-coated blueberries into the cake batter using a rubber spatula or large spoon. Distribute them evenly but mix minimally.

8. Bake the Cake(s):

• Pour the batter into the prepared pan(s).
  • For 9×13 Pan: Spread batter evenly. Bake for 30-35 minutes.
  • For 3x 8-Inch Pans: Divide batter evenly. Bake for 25-30 minutes.
• The cake(s) are done when the top looks set, springs back lightly when touched, and an inserted toothpick comes out clean or with a few moist crumbs.
• Cool the cake(s) in the pan(s) on wire racks for about 10 minutes (for layers) or longer (15-20 min for 9×13) before removing.
• For Layer Cake: Carefully run a thin knife around edges and invert layers onto wire racks to cool completely. Peel off parchment.
• For 9×13 Cake: Let cool completely in the pan.

9. Make the Cream Cheese Frosting: (Use appropriate quantity based on cake format)

• Ensure butter and cream cheese are properly softened.
• In a large bowl using an electric mixer, beat the softened butter until smooth and soft.  
• Add the softened cream cheese and beat together until well combined and smooth, scraping down the bowl.
• Beat in the fresh lemon juice.
• With the mixer on low speed, gradually add 3 cups (or 4.5 cups for layer cake) of the sifted powdered sugar, mixing until incorporated.
• Check consistency and sweetness. Beat in the remaining powdered sugar (up to the max listed for your version) about 1/2 cup at a time until the desired sweetness and a thick, spreadable consistency is reached.
• If the frosting seems too thick at any point, beat in 1 tablespoon of whipping cream (or up to 2 tbsp for layer cake version) until smooth and fluffy.

10. Decorate and Serve:

• For 9×13 Cake: Once completely cool, spread the frosting evenly over the top of the cake directly in the pan. Optionally decorate with blueberries and lemon slices. Slice and serve.
• For Layer Cake: Ensure layers are fully cooled (chilling wrapped layers makes assembly easier). Level tops if needed. Place one layer on a serving plate. Frost the top (~3/4 cup). Add second layer, frost top (~3/4 cup). Add third layer (upside down for flat top). Apply a thin crumb coat of frosting to sides and top; chill 20-30 minutes. Apply remaining frosting smoothly or decoratively to sides and top. Optionally, decorate with blueberries and lemon slices.
• Slice layer cake using a thin, sharp knife dipped in hot water and wiped clean between cuts. Serve cool room temperature or chilled.