In the bowl of a stand mixer, place the flour, xanthan gum, granulated sugar, yeast, cream of tartar, cardamom, cinnamon, and allspice and whisk to combine well. Add the salt, lemon zest, and orange zest, and whisk once again to combine well. In a separate small bowl, place the dried currants and toss them with a tablespoon of the dry ingredients and set the bowl aside.
Add the vanilla, vinegar, milk, 1 egg and the egg yolk, and the butter to the stand mixer and mix on low speed to combine. Turn the mixer up to high speed, and allow to mix for about 6 minutes uninterrupted. Turn off the mixer and add the dried fruit and reserved dry ingredients to the dough, and mix until the currants are evenly distributed throughout the dough.
Turn the dough out onto a lightly floured piece of parchment paper and sprinkle lightly with more flour. With well floured hands and a bench scraper, divide the dough into 11 pieces. With well-floured hands, roll each piece of dough into a tight ball. Place the buns about 2 inches apart on parchment-lined baking sheets.
Place in a warm and humid, draft-free environment to rise until about 140 percent of their original size. They will rise more during an "oven spring."
While the buns are rising, preheat the oven to 375 degrees and make the egg glaze. In a small bowl, place the remaining egg and beat it well. Add the 4 tablespoons of confectioners' sugar and beat to combine well.
Once the dough has finished rising, brush the top of each bun generously with the egg glaze. With a sharp knife, slice a shallow cross on the top of each roll. Place the baking sheet in the center of the preheated oven and bake, rotating once during baking, until the buns are golden brown and firm to the touch, 25-35 minutes. Allow the buns to cool for about 10 minutes on the baking sheet before transferring them to a wire rack to cool completely.
While the buns are cooling, make the icing for the cross. In a medium-sized bowl, combine the remaining 1 cup confectioners' sugar with 2 tablespoons milk. Mix to combine well. It will form a thick paste. If it is so thick that you will not be able to squeeze it through a plain pastry tip, thin it with some more milk, a drop or 2 at a time. It should be just thin enough to pipe, but still very thick. Once the rolls have cooled completely, place the icing in a pastry bag fitted with a #10 plain tip, and pipe a cross neatly over the cross you made with a knife, on each roll. If you attempt to ice the cross before the rolls are completely cool, the icing will melt and run.