Cake:

Preheat Oven:
Preheat oven to 350°F (175°C). Line a 10×15-inch sheet pan (jelly roll pan) with parchment paper, including up the sides. Spray lightly with nonstick spray.

Mix Dry Ingredients:
In a bowl, whisk together cake flour and baking powder. Set aside.

Cream Butter & Sugar:
In a stand mixer fitted with a paddle attachment, cream butter, sugar, and vanilla extract until light and fluffy (about 5 minutes).

Add Egg Whites:
With mixer on low, add egg whites one at a time, mixing well after each addition.

Alternate Dry & Milk:
Add ⅓ of the flour mixture, then half the milk, another ⅓ flour, remaining milk, and final flour portion. Mix just until combined.

Bake:
Pour batter into prepared pan and bake for 28–30 minutes, or until a toothpick inserted in the center comes out clean. Cool completely.

Chill Cake:
Once cooled, chill in freezer for 10–15 minutes (or refrigerate 1–2 hours) to firm up for easier assembly.

Blueberry Topping:

In a saucepan over medium heat, combine 4 cups blueberries, sugar, lemon juice, and cornstarch.

Cook until thick, bubbly, and berries begin breaking down.

Remove from heat and cool completely.

Stir in remaining ½ cup fresh blueberries. Refrigerate until ready to use.

Whipped Cream Cheese Topping:

Chill mixing bowl and whisk attachment for 15 minutes.

Beat cream cheese and sugar until smooth.

Slowly add heavy cream and mix until soft peaks form.

Scrape bowl and continue mixing until stiff peaks form.

Add vanilla and salt; mix gently until combined.

Chill until ready to assemble.

Assembly:

Remove cake from pan using parchment edges.

Slice cake evenly in half to create two rectangles.

Place one layer on serving platter.

Spread half of the blueberry topping over the layer.

Add ⅓ of the whipped topping.

Place second cake layer on top.

Spread remaining whipped topping and top with remaining blueberries.

Serve immediately for best texture.