In a medium bowl, dissolve the yeast in the soy milk and add 2 tbsp. of the flour. Whisk to incorporate and let rest in a warm place for 20 min.
In the bowl of a mixer, with the dough hook, combine the dry ingredients: the rest of the flour, salt, sugar, and orange zest. Mix.
In a small bowl, whisk together the orange juice, mashed potato, and yeast-flour mixture. Add to bowl with the flour mixture and mix on low until the dough begins to incorporate.
Add the ½ cup + 1 tbsp. of softened butter little by little and increase speed to medium. Mix for 15 min. until the dough has come off the sides of the bowl and is stretchy but not sticky.
Place the dough in a large oiled bowl, cover with a towel and let rise for 1 to 1 ½ hours, or until doubled in size.
Punch down the dough and lay on a floured surface. Roll out the dough into a log shape and bend the ends around to form a circle. Press edges together to avoid separation.
Place the rosca on a parchment lined sheet tray and cover with a towel and let rise for 1 hr. in a warm place (70- 75F) or until double in size.
Meanwhile preheat the oven to 350F.