Start by placing the three oranges in a sauce pot; cover them with water and simmer gently for 90 minutes, keeping the oranges covered with water the whole time.  Then, remove the oranges from the water, cut in half and remove seeds, if any.  Place the oranges (both pulp and skin) in the blender and blend until smooth.  Let cool.

While the oranges are blending, cream the eggs with the sugar.  Add the optional teaspoon of almond or vanilla extract and measure out the almond flour and the baking powder.  Mix the pureed orange with the yogurt and blend into the creamed sugar and eggs.  Once the two are incorporated, gradually add the almond flour and baking powder - do this gradually.

Once mixed, pour the batter into a parchment-lined and greased half-sheet tray or baking pan - making sure that the sides are at least 3/4" tall.  Place into a 350 degree oven (170C) for 40 minutes or until done.  Let cool fully before removing from the baking pan. 

The cake should be dense, but moist and tender.  Serve with garnishes of your choice:  flavored whipped cream, ice cream, shaved chocolate, and/or peeled slices of oranges and mandarins.