1
Mix all the ingredients in a bowl (saving the two egg whites), in the listed order, then grate the cold butter straight from the refrigerator and add it to the bowl.
2
Mix with the hands just enough to make a dough, but don't overmix. Form a ball, then flatten it into an 8x8 inch square on a plastic wrap, wrap it around and place it in the refrigerator for one hour.
3
Make the dry nut and fruit mix. Roughly chop the nuts (walnuts, pecans, pistachios, blanched almonds) and the bigger dry fruits, the dark chocolate, and add them to a mixing bowl with the rest of the ingredients.
4
Add tapioca flour, and mix well, so all the pieces are coated with the flour.
5
Add the cold dough to the pan and spread it out. Push it up the edges for about 1/2 inch or less. Cover the dough with plastic and put it back in the fridge for another 30 minutes,
6
Separate the yolks from the whites. Add the yolks to a big mixing bowl, mix it with the sweetener, and beat it until creamy. Beat the 5 egg whites in a separate bowl until stiff.
7
Slowly incorporate the egg whites into the egg yolks, gently folding them in with a spatula. Then add the nut and fruit mix, folding them in until they are evenly distributed (don't overmix).
8
Add the filling on top of the dough, level with a spatula, and bake for 10 minutes at 350F/180C. Turn the heat down to 320F/160C and bake for 35 more minutes.
9
When it is done, take the baking sheet out, and let it rest, at room temperature, in the baking sheet, for about 6 hours. Refrigerate before slicing.