To prep the yeast:
Mix the yeast with the teaspoon of sugar stirring into the warm water (110 to 115-degrees). Set aside to allow the yeast to proof for about 5 minutes.
For the bread dough:
In a large bowl, stir the warm milk together with the vegetable oil, sugar, and orange zest.
In a separate bowl, whisk 1 cup of the flour with the kosher salt, cinnamon, cardamom.
Add the flour and spices along with the yeast mixture, blending into the milk mixture on low speed.
Add the 3 whole eggs, beating in on low until blended together.
Slowly add 3 cups of flour, 1 cup at a time using low speed, then stir in the mashed potatoes.
Blend the currants and raisins into the dough.
Scrape the dough down into the bowl and cover with plastic wrap or a kitchen towel. Let rise in a warm place until doubled in size, about 1 to 1-½ hours.
Dissolve the baking powder and baking soda into the water. Punch the dough down and stir the baking powder and baking soda mixture into the dough.
Turn out onto a surface that has been generously floured. If the dough is still sticky, work the additional flour in, kneading as you go. You want a soft dough, but not one that is sticky.
Roll or pat the dough to a thickness about ½” thick. Using a 2-½" to 3” biscuit cutter, cut out rounds of dough and shape into rolls, tucking the edges under.
Place the buns in a baking pan that has been sprayed with a cooking spray. You want the rolls to have room to rise, it’s fine if they’re touching slightly. Lightly beat the egg white and brush the tops of each roll. Cover the pans with a plastic wrap and refrigerate overnight, they’ll rise as they chill.
When you’re ready to bake and let the rolls rest at room temperature for about 20 minutes, just to take the chill off.
Bake in a 375-degree oven for 25 minutes or until golden in color. If the rolls begin to brown too much on top before they’re baked through, cover loosely with a sheet of foil. The rolls should sound hollow when tapped and the bottoms should be lightly golden in color when done.
Allow to cool while you make the frosting.
For the frosting:
Whisk the powdered sugar with the vanilla extract and milk. Place the frosting into a piping bag or bottle and pipe a cross over the tops of each.