In a standing mixer, cream on high speed the butter with the sugar until pale and frothy.
Slowly add the egg yolks to the mixture and beat until combined.
In a different bowl, whisk the egg whites into meringue for approximately 15 minutes, until double in size and soft peaks form.
Fold the meringue into the butter mixture. Add the orange zest, milk and mix well.
Sift the all purpose flour with the baking powder and mastic into the mixture. Using the paddle attachment, beat on medium speed until homogenous.
Preheat the oven at 320oF / 160oC.
Transfer the mixture into a buttered and floured 11'' / 30 cm round baking tin. Sprinkle the almond flakes on top.
Tap the baking tin on the cooking bench two-three times to evenly distribute the cake mixture.
Bake for 50 minutes.
Remove the cake from the oven and allow to cool down for 10 minutes.
Turn the cake upside down and insert a coin covered in tin foil through the base of the cake. Transfer the cake into a platter.
Dust with powdered sugar. Serve in slices.