Preheat the oven to 350°. Grease one 8 inch round cake with nonstick cooking spray. Pour the melted butter into the pan and sprinkle with the brown sugar. Arrange the sliced clementines over the butter and sugar. Set aside while the batter is prepared.
In a small bowl combine the sugar with the zest of the clementines until fragrant.
In a large bowl whisk together the clementine sugar, flour, baking powder, baking soda, and salt.
In another bowl whip the egg whites until medium-stiff peaks form. Fold ⅓ of the egg whites into the cake batter to loosen it up, followed by the remaining egg whites. Fold just until no streaks of egg whites remain and the batter is fully combined.
Pour the batter into the cake pan over the sliced clementines. Bake for 30-35 minutes, just until the center of the cake springs back when lightly pressed on or a toothpick comes out clean.
Let the cake cool for about 10 minutes before inverting it onto a plate or cake stand. It is best served immediately but can be wrapped in plastic and stored for up to 3 days at room temperature.