Roast the nuts if need be. Place on a dry baking sheet. Place in a 350 degree oven for 15-20 minutes.
Melt the sugar in a small sauce pan over low-medium heat. Stir consistently, but the entire time is not needed. Melt until amber in color and all sugar grains are dissolved. I use a spatula to stir.
While waiting for the sugar to melt, place the roasted (unsalted) nuts (all hazelnuts and almonds - 2 cups in total), on a parchment lined baking sheet. Spread to a single layer.
Immediately upon finishing the melting of the sugar, pour as evenly as possible over all of the nuts. Not all of the nuts will be entirely covered, and that is okay, but make sure it is evenly and lightly distributed so the nuts receive the melted sugar (caramel), not the pan.
Set aside the nuts to cool while you make the chocolate filling.
Rough chop the chocolate (9 ounces) you will be using. Add the chocolate and the butter to a small to medium sauce pan. Melt over low-medium heat until combined. Add the salt and espresso (optional). Remove from the heat to let cool while you whip the egg whites.
In a small mixing bowl, mix with a whisk or hand mixer the egg whites until soft white peaks form. Add 1/3 of the egg whites gently to the melted chocolate that has now cooled. Fold to keep as much air as possible. Then add the next 1/3 of egg whites, fold and then the final 1/3 of the egg whites until combined, but no more.
Using a food processor, break of the nuts and pulse until roughly (or finely - your preference here) chopped.
Add 1 1/3 of the nuts to the chocolate filling. Fold in with the spatula until combined.
Take the prepared pastry crust and pour in the chocolate and nut mix. Spread evenly. Sprinkle the remaining 2/3 of chopped nuts to the top of the tart.
Refridgerate for at least 3-4 hours.
Enjoy chilled with a cup of hot tea.