Combine all ingredients for the Sponge and mix until dough is formed. Set aside and ferment overnight.
For the Dough: Combine flour and yeast then set aside. In a mixing bowl, dissolve sugar and salt in fresh milk. Stir in egg yolks. Add flour mixture, knead lightly. Add the fermented sponge and mashed potato then mix until dough is formed. Add butter. Continue kneading until fully developed. Rest dough.
Cut and scale into desired weight (15 grams). Form into balls. Flatten balls with a rolling pin. Roll to form into small batons. Form into spirals and place in greased ensaymada molds.
Proof until double in volume. Bake at 350°F for ten minutes or until done.
Cream butter until light. Dissolve sugar in milk mixture. Gradually add milk mixture in butter while creaming. Continue creaming until butter is light and fluffy. Use as frosting for ensaymada. Top ensaymada with grated cheese.