Preheat oven to 180ºC (175 C). Sift all-purpose flour, salt, potato flour and vanilla sugar into a medium-size bowl. Cut in butter until mixture forms coarse crumbs; mix in raisins. Mix together to form a soft dough. Roll out dough on a floured surface to a rectangle slightly smaller than an 28 cm x 18 cm baking pan. Place rolled-out dough in pan; press to fit. Smooth top; prick well. Bake about 25 minutes or until very lightly browned. Cool a few minutes. Using a sharp knife, mark through surface of mixture with lines to use as a guide for cutting. Let cool in pan. Cut mixture in 20 squares; remove from pan. Place chocolate in a small bowl over a pan of gently simmering water; stir until melted and smooth. Line a baking sheet with foil. Dip bars in chocolate, coating evenly; lift out with a fork and tap gently on side of bowl to remove excess chocolate. Place on foil. Place baking sheet in a cool place until chocolate sets. If desired, any remaining chocolate can be piped over bars for decoration.