Preheat your oven to 350°F (175°C) and prepare a 9-inch round cake pan with non-stick spray.

In a medium saucepan, combine 1/2 cup of brown sugar and 1/4 cup of unsalted butter over medium heat until syrupy, about 3-5 minutes. Pour into the pan and add blueberries on top.

Beat 2 egg whites in a clean bowl on high speed until stiff peaks form, about 3-5 minutes.

Cream together 1/2 cup of softened butter and 1 cup of sugar until fluffy, about 3-4 minutes. Add in the 2 egg yolks and 2 teaspoons of vanilla extract. Mix well.

In a separate bowl, whisk together 1 1/2 cups of flour, 2 teaspoons of baking powder, and 1/2 teaspoon of salt. Gradually add this dry mixture and 3/4 cup of whole milk to the butter mixture, mixing until just combined.

Gently fold in the beaten egg whites until no visible streaks remain.

Spread the batter evenly over the blueberries and caramel in the pan, leveling the surface.

Bake in the preheated oven for 55-60 minutes, until a toothpick comes out clean and edges are golden brown.

Let cool for 15 minutes, then carefully invert onto a serving platter and tap to release.