Grease a 9-in square pan with a little butter.

Grate the zest from all the fruits.

 Put 2 tsp of the zest in a large bowl with the butter and sugar. Using an electric mixer, cream the butter and sugar until light in color and fluffy.

  Add the eggs one at a time, and mix the batter after each one.

 In a medium bowl, mix together the flour, corn meal, salt and baking powder.  

Sift half the mixture over the butter mixture and blend well. 

 Fold the yogurt into the butter mixture and sift the remaining flour mixture on top and fold well.

 Scrape batter into prepared pan and tap gently. 

  Bake for 20 minutes after which turn temperature down to 325F and bake for an additional 15 minutes or until toothpick inserted into center comes out clean.

  Juice 1 lemon, 1 lime,1 mandarin and remaining zest into a bowl, mix in the sugar without letting in dissolve.