Preheat the oven to 350ºF. Gently cream the sugar, butter and salt together, taking care not to over-mix. Add the pastry flour, bread flour, and vanilla powder and mix until just incorporated, taking care not to over-mix. Chill the dough until ready to bake. Roll the dough to a ¼-inch sheet, put on a lined sheet tray and bake for approximately 10 minutes. Remove from the oven, cool, and cut into rectangles.