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

In a small saucepan over medium heat, melt the brown sugar and butter together until the mixture is syrupy and the sugar is dissolved.

Pour this mixture into the bottom of the prepared cake pan, tilting as needed to spread evenly. Add the fresh blueberries over the syrup in a single layer.

Separate the two eggs, placing the whites in a clean bowl. Beat the whites with a mixer on high speed until stiff peaks form and set aside.

In a large bowl, cream the softened butter and sugar together until light and fluffy. Beat in the egg yolks and vanilla extract until well combined.

In a separate bowl, mix together the flour, baking powder, and salt. Add these dry ingredients to the butter mixture, alternating with the milk, mixing just until combined.

Carefully fold the beaten egg whites into the batter with a spatula, being gentle to keep the mixture light. The batter will be quite thick.

Drop large spoonfuls of the batter over the blueberries in the pan, then gently spread the batter evenly without disturbing the fruit.

Bake for 55–60 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan on a rack for 15 minutes.

Run a knife around the edge to loosen the cake. Place a serving platter over the pan and carefully invert the cake onto the platter.

Leave the pan on top for another 5–10 minutes so the blueberry juices soak in. Remove the pan.

Serve warm or at room temperature—delicious with whipped cream or vanilla ice cream!