In food processor, combine flour, sugar and salt; add cold butter and process very briefly until coarse, 15 sec. Separately, beat yolks and water together. Add egg mix to running processor in slow stream. Pulse briefly until dough comes together (do not mix more than 30 sec or friction will heat/break butter). If too crumbly, add a few drops of water. Turn dough out onto lightly floured work surface and quickly shape into a rough disk. Wrap with plastic and chill at least 1 hour before use. May be frozen up to one month in advance: thaw overnight in refrigerator.
Chocolate Hazelnut Filling
Heat oven to 400 degrees. On lightly floured surface, roll out sugar crust to 14" rough circle (some cracks are normal). Fit pastry into tart pan, pressing dough into fluted edges. Trim excess and use to fix any breaks. Chill 30 min. In a large bowl, whisk eggs, sugars, Kahlúa and vanilla until frothy and pale, 20 sec. Whisk in dry ingredients and butter.
Stir in chocolate and pour into tart shell. Add nuts and press gently into filling. Bake 10 min. Reduce heat to 350 degrees. Lay a piece of foil flat over tart (do not crimp edges or cover tightly), and bake 40-50 min more, until center begins to rise slightly and pick inserted in center comes out clean. Cool in pan to room temp before releasing tart and slicing to serve. Add a spoonful of whipped cream, if desired.