In a food processor, combine the cold butter, flour and salt. Pulse together until you get a fine crumb with a few pea-sized lumps of butter. Drizzle in 2 tbsp of the cold water and pulse again. Pinch some of the mixture between your fingertips, if it holds together it’s done. If it doesn’t drizzle in another tablespoon or two of water and pulse again.
Tip out onto a piece of plastic wrap, press together into a ball and press down into a disk. Wrap in the plastic wrap and chill for 1 hour.
Make the filling:
Meanwhile wipe out the food processor. Place the 2/3 cup of hazelnuts into the food processor with half of the sugar and blend until very fine in texture. Add the butter and blend again until smooth. Add the egg whites, flour, baking powder and salt. Blend again until smooth.
Mix the mashed banana with the sugar and lemon juice in a small bowl. Set aside.
Blind bake the pastry:
Preheat the oven to 350oF.
Roll the chilled pastry out between two pieces of baking paper to just slightly larger than an 8-inch tart tin. Peel off the top layer of baking paper, flip the pastry into the tin and remove the other layer of baking paper. Gently press the pastry down into the tin and cut off any excess around the edges.
Crumple up one of the pieces of baking paper you were using, un crumple it and lay it into the lined tart tin. Fill with baking beans and bake for 10 minutes with the baking paper inside. Lift out the baking paper and baking beans and cook the pastry for a further 5 minutes.
Fill and bake:
Remove the tart tin from the oven, spread with the mashed banana then cover with the hazelnut mixture you made earlier and top with chocolate chips and the extra hazelnuts. Bake for 25-30 minutes until golden and a toothpick inserted into the centre of the tart comes out clean. Let cool for 10 minutes before slicing and serving.