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.
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.
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.
Add tapioca flour, and mix well, so all the pieces are coated with the flour.
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,
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.
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).
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.
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.