Preheat oven to 350 F (175 C). Grease several baking sheets with butter. In a large bowl, beat butter with sugar until creamy. Beat in egg yolks, orange flower water and milk. Add currants. Sift flour and salt into bowl; mix in to form a fairly stiff dough. Knead lightly on a floured surface until smooth. Roll out to 1/8-inch thickness. Using a round 2-1/2-inch-fluted cookie cutter, cut out circles from dough; place on prepared baking sheets. Knead and roll out trimmings; cut out more circles. Continue until dough is used up. Bake cookies 10 minutes. Remove from oven. Brush cookies with beaten egg white; sprinkle lightly with superfine sugar. Return to oven for about 5 minutes longer or until lightly browned. Remove from baking sheets to wire racks; cool. Store in an airtight container.