In a small sauce pan, warm the buttermilk. Do not boil! It only needs to be warm enough to activate the yeast.
Remove the buttermilk from the heat, and add the yeast. Let it sit for 5-10 minutes.
In a separate bowl, whisk together the flour, xanthan gum, cream of tartar, salt, cinnamon, and nutmeg. Set aside.
Cream together the sugar, butter, and orange zest.
Blend in the eggs.
Alternate adding both the flour mixture and the buttermilk to the butter mixture, beginning and ending with dry ingredients.
Continue to mix until the dough pulls away from the sides of the bowl and is smooth.
Add the currants to the dough and mix until they're evenly dispersed throughout the dough.
Cover the dough and allow to rest for about 1 hour. It won't rise much.
Add parchment paper to a rimmed baking sheet. Divide the dough into 12 equal parts.
Working on a lightly floured surface, shape each part into a ball and place it on the baking sheet, leaving about 1 inch between each bun.
Cover and let rise in a warm location until almost doubled in size, about 1 hour.
When the buns have risen, pre-heat the oven to 375ºF.
With a sharp knife, slash a cross in the top of each bun, about 1/4" deep.
Brush each bun with the egg white.
Bake at 375ºF for about 30 minutes, 'til the tops of the buns are golden brown.
Once baked, transfer the buns to a wire cooling rack.