In a small bowl stir together milk, yeast, and 1 teaspoon granulated sugar.
Let mixture stand 5 minutes, or until foamy.
Into a large bowl sift together flour, allspice, cinnamon, salt, and remaining 1/2 cup granulated sugar.
Cut butter into bits and with your fingertips or a pastry blender blend into flour mixture until mixture resembles coarse meal. Lightly beat 1 whole egg with egg yolk. Make a well in center of flour mixture and pour in yeast and egg mixtures, currants, raisins, and zests. Stir mixture until a dough is formed.
Transfer dough to a floured surface and with floured hands knead until smooth and elastic, about 10 minutes.
Transfer dough to an oiled large bowl and turn to coat.
Let dough rise, covered with plastic wrap, in a warm place until doubled in bulk, about 1 1/2 hours.
Butter 2 large baking sheets.
On a floured surface with floured hands knead dough briefly and form into two 12-inch-long logs.
Cut each log crosswise into 12 equal pieces. Form each piece into a ball and arrange about 1 1/2 inches apart on baking sheets.
Let buns rise, covered, in a warm place until doubled in bulk, about 45 minutes.
Preheat oven to 400°F.
While buns are rising, lightly beat remaining egg with superfine sugar to make an egg glaze. On a lightly floured surface with a floured rolling pin roll out pastry dough into a 20- by 6-inch rectangle (about 1/8 inch thick). With a sharp knife cut rectangle crosswise into 1/8-inch- wide strips.
Brush buns with egg glaze and arrange 2 pastry strips over center of each bun to form a cross. Trim ends of pastry strips flush with bottoms of buns.
Bake buns in upper and lower thirds of oven, switching position of sheets halfway through baking, until golden, about 12 minutes.
Transfer buns to a rack to cool slightly. Buns may be made 1 week ahead and frozen, wrapped in foil and put in a sealable plastic bag. Thaw buns and reheat before serving.
