In a medium bowl, combine the water, yeast, and ½ teaspoon sugar. Set aside to proof.
In the bowl of a stand mixer fitted with the hook attachment, add the remaining ⅓ cup of sugar, flour, mashed potatoes, salt, eggs, and butter. Turn the mixer on low until combined.
Slowly pour in the proofed yeast mixture and increase the mixer to medium. Continue on medium until a smooth dough forms and no longer sticks to the walls of the bowl.
Transfer dough to a large greased bowl and cover loosely with plastic wrap. Let rise for 90 minutes in a dark and warm environment, until doubled in size.
Spray a 9x13-inch baking pan with cooking spray. Divide and roll the dough into 16 balls, weighing about 85 grams each, and place in the greased baking pan, seam-side down. Cover loosely with plastic wrap and allow to rise for 2 hours in a dark and warm environment.
During the last 20 minutes, preheat oven to 350°F. Remove the plastic and bake for 20 to 25 minutes, until the tops are golden brown.