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 brown sugar and 1/4 cup butter over medium heat, stirring until syrupy, about 3-5 minutes. Pour into the prepared pan and arrange blueberries on top.

Beat 2 egg whites in a clean bowl until stiff peaks form, about 3-5 minutes. Set aside.

In a large bowl, beat together 1/2 cup softened unsalted butter and 3/4 cup granulated sugar until light and fluffy, about 4-5 minutes. Add 2 egg yolks and 1 teaspoon vanilla extract, mixing well.

In another bowl, whisk together 1 1/2 cups all-purpose flour, 1 1/2 teaspoons baking powder, and a pinch of salt. Gradually add this to the butter mixture, alternating with 1/2 cup milk, mixing until just combined.

Fold the beaten egg whites into the batter until no white streaks remain visible.

Spoon the batter over the blueberries in the cake pan, spreading gently to keep the fruit undisturbed.

Bake in preheated oven for 55-60 minutes, or until a toothpick inserted comes out clean.

Let the cake cool in the pan for 15 minutes before inverting onto a serving platter.