Divide pie crust into 8 equal pieces by cutting it in half, then in half again. Roll out each piece into a rough 6 inch circle and stack them up on parchment paper. Refrigerate for two hours minimium. This is a great step to do a day ahead.
Preheat oven to 350°.
Rub loose skins off hazelnuts, leaving any that cling. Grind hazelnuts and ⅓ cup sugar in the food processor into a fine paste, 2-3 minutes. Add butter, flour, Frangelico, salt, and one egg. Pulse until smooth and set aside.
Slice your plums into ¼ inch wedges and toss with ½ cup sugar and Frangelico.
Beat the other egg with 1 tsp. of water and set on the table next to all the other components. Line a large cookie sheet with parchment paper and have a pastry brush handy.
Add a heaping tablespoon to the centre of a disc of dough and spread in a circle, leaving a 1" border. Brush the border with egg wash. Spoon some fruit onto the frangipane, and fold the edges in, tucking each fold into the next, making little nests for your fruit fillling.
Brush egg wash over outer ring of pie dough, and sprinkle with large crystal sugar, if you have any. Otherwise regular sugar will do.
Pop them into the oven and cook for 35-35 minutes, checking after 30 minutes. The crust should be golden brown and the plums and blackberries entirely softened.
Cool on a wire rack for one hour. Serve with ice cream, if you like. Or just eat out of hand, without even taking the time to get a plate.