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

In a medium saucepan, combine brown sugar and butter over medium heat, stirring until syrupy, about 3-5 minutes. Pour it into the cake pan.

Evenly distribute the blueberries over the caramel mixture in the pan.

Beat the egg whites in a clean bowl until stiff peaks form, about 3-4 minutes, then set aside.

In another bowl, cream together softened butter and granulated sugar until light and fluffy (4-5 minutes). Add egg yolks and vanilla extract.

Combine flour, baking powder, and salt in a bowl. Gradually add this to the wet ingredients, alternating with milk.

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

Spoon the batter over the blueberries in the pan and spread gently.

Bake for 55-60 minutes or until a toothpick comes out clean.

Cool in the pan for 15 minutes, then invert onto a platter.