Combine 1 1/2 cups of the flour, salt, sugar and butter in food processor. Pulse until it resembles coarse cornmeal.
Whisk egg, vanilla, water vinegar and oil in a small bowl. Add the wet ingredients and remaining flour to the food processor, alternating between the two and mix until just blended.
Split dough in half, wrap in plastic discs and refrigerate for at least 1 hour or overnight.
Preheat the oven to 350º and line two baking sheets with parchment paper and set them aside.
On a floured surface, working with one dough disc at a time, roll it out until about 1/8 inch thick, adding more flour as needed to prevent sticking.
With a cookie cutter, mason jar lid or water glass, cut out 3 inch circles.
Place a 1 teaspoon of prune filing (or half nutella and half raspberries), leaving a border around the edges. Brush the border with egg wash and fold the edges up to form a triangle shape, pinching the corners firmly to seal. Sprinkle with a tiny pinch of flaky salt.
Re-roll scraps of dough and repeat. Place cookies on the baking sheets, about an inch apart and bake for about 15 minutes.