In a small bowl, add the yeast and 2 teaspoon sugar to the water (be sure that the water is 110-115°F). Give it a stir and set aside.
1 packet (or 2 1/4 teaspoons) quick rise yeast, 1/3 cup + 2 teaspoons sugar, 1/4 cup warm water
In the bowl of a stand mixer fitted with the dough attachment, add the warm milk, ⅓ c sugar, mashed potatoes, butter, egg yolk, salt and vanilla. Stir until the butter is melted.
1/3 cup + 2 teaspoons sugar, 2/3 cup very warm milk, 1/3 cup mashed potatoes, 1/4 cup butter, 1 egg yolk, 1/2 teaspoon salt, 1 teaspoon vanilla
Add 1 cup of the flour and stir to combine.
3 1/2 cups all-purpose flour
Pour in the yeast mixture (should have risen a bit and look foamy) and stir.
Add the remaining flour. Knead the dough (by hand for 12 minutes) or by using the stand mixer on low speed for 10 minutes. The dough will be a bit sticky, but you should be able to touch the dough lightly with your finger and have it not stuck to your finger. (It might pull a bit, but should release from your finger). If it's too sticky, add an additional ¼ - ½ cup flour and knead an additional few minutes to make sure its well-incorporated.
Using greased hands, gather the dough into a ball and place into a greased glass bowl. Cover with plastic wrap and set in a warm place to rise for 60-90 minutes.