In a large bowl, combine flour, ground almonds, icing sugar and orange zest. Using a pastry blender (I used food processor), cut in butter until mixture is formed to a soft dough. Press evenly into the bottom of a 8 inch x 2 square (lined) baking pan.
Bake for 20 minutes, or until light browned.
In a medium bowl, whisk together sugar, eggs, orange juice and zest. Combine plain flour and baking powder, add to sugar mixture, whisking to combine. Pour over crust.
Bake for 20-25 minutes, or until just set. Sprinkle with confectioners sugar and decorate with a piece of mandarin orange on top.
Recipe by Anncoo Journal at https://www.anncoojournal.com/melt-in-your-mouth-orange-squares/