Preheat the oven to 180°C. In a mixer, beat the egg yolks and half the sugar until you have a frothy, off-white mixture.
In a blender, powder the almonds, mastic, and 1 tsp sugar.
In a separate bowl, beat the egg whites with the rest of the sugar until you have a meringue. Add the meringue little by little to the yolk mixture, alternating with semolina, vanilla extract, and almonds, softly mixing until smooth. Finally, add the melted butter and mix a little more until smooth.
Grease a medium-sized baking tin with butter, sprinkle with some semolina, and pour the mixture into the tin. Sprerad the mixture evenly using a spatula, and decorate with almond flakes. Place the tin in the bottom oven shelf and bake for about 50 minutes until golden.
Upon removing from the oven, cut into pieces and leave to cool completely.
Prepare the syrup in a pan, adding all the ingredients except the butter, and boil for 7 minutes. Once boiled, add the butter, mix, and pour over the cake, spoonful by spoonful. Let the cake absorb all of the syrup and cool down. Cut into slices and serve on its own or with kaimaki ice cream.