Preheat the oven to 350 degrees F [180 degrees C]. Make sure the egg whites are at room temperature before beginning the recipe.
Spray an 8-inch [20-cm] round cake pan with cooking spray. Cut a round of parchment or waxed paper to fit the bottom of the pan. Smooth the paper into the pan, spray with cooking spray, and coat the bottom and sides with flour. Set aside.
Combine the cocoa and brown sugar in a small bowl. Pour in the boiling water and stir until smooth. Set aside to cool.
In a large bowl, whisk together the yogurt, oil, corn syrup, applesauce, egg yolk, vanilla and orange zest. Add the chocolate mixture and mix thoroughly.
In a separate bowl, combine the cake flour, cornstarch, baking powder, soda and salt. Stir the dry ingredients gently into the chocolate mixture.
Make sure the bowl and whip attachment of your electric mixer are perfectly clean and grease-free. Beat the egg whites until soft peaks form. Sprinkle the sugart slowly over the top, one tablespoon at a time, and continue to beat until the whites are stiff but not dry.
Stir a large spoonful of beaten egg whites into the batter to lighten it. Gently fold in the rest of the egg whites in two steps.
Pour the batter into the prepared pan and bake for 35 to 40 minutes, or until a toothpick or cake tester inserted in the center comes out clean. Cool on a rack for twenty minutes, then tip out of the pan onto another rack to cool completely. The cake will fall slightly as it cools. This cake is best served the same day it is baked, but if you allow it to cool completely and then wrap it carefully, it will be find the next day.
To prepare the sauce:
Yogurt Cheese: Place 1 1/2 cups yogurt in a strainer lined with a coffee filter, muslin or paper towel. Place the strainer over a bowl. Refrigerate overnight. The liquid will drain from the yogurt and you will be left with a creamy cheese mixture.
In a medium bowl, whisk together the yogurt cheese, milk, orange juice concentrate, syrup, orange zest and lemon juice. Keep whisking until the sauce is perfectly smooth. This sauce can be made ahead and refrigerated.
To serve: Cut the cake into eight wedges. Place a wedge on each dessert plate and top with a spoonful of sauce. Lay a few orange segments and a sprig of mint on the side. [Leftover cake will keep for a few days if wrapped in plastic.