Preheat the oven at at about 350 degrees Fahrenheit (180 degrees Celsius).
Line the base of a 9-10 inch springform pan with wax paper and grease the sides.(You can use an even wider pan for a thinner cake-it will take less time to bake)
In a bowl mix the flour and baking powder and set aside.
In a bowl, cream the sugar and butter.
Add the orange juice, vanilla and orange zest- mix with a mixer about 2 minutes.
Whip the eggs in a small bowl and add to the butter mixture and mix for another 2 minutes.
Add the flour mixture to the wet ingredients and mix for 2 minutes at low speed-do not over mix. If you are adding the coin in the batter, add it now.
Pour batter (it will be thick) in the pan, smoothing the top with a spatula.
Bake for about 45-60 minutes. Check with a thin sharp knife in the center, it should come out clean. If not bake for additional time.
Remove from oven and let cool completely.
Release the pan and turn cake upside-down on another plate. If putting the coin in now, wrap in foil and push it through the cake. Turn cake back on top and sprinkle with powdered sugar, you can also even out the top of the cake slicing with a long knife. You can make a design using paper cut outs or using almonds or raisins or you can ice the cake with a simple icing made with powdered sugar and water. Make sure you have a lucky charm for the person who will get the coin in their piece.