Grease or line a baking sheet with parchment paper and preheat the oven to 450F.
Stir flour, potato starch, and potato flour together in a large bowl.
Add the milk, whole eggs, and yeast to the milk mixture; stir well.
Put the yeast mixture into a separate small bowl and add the sugar; let it stand for 5 to 10 minutes, until foamy (be sure to see the bubbles on top). Add the softened butter to the shortening in another bowl and beat until blended (or use an electric mixer).
In a separate bowl, combine the remaining 2 tablespoons of sugar and salt with the butter/shortening mixture.
The butter/shortening mixture and yeast mixture should be mixed in a large bowl.
Stir in the flour mixture just until combined. It's best to knead the dough by hand or with an electric mixer to prevent sticking.
Put the dough in a plastic bag and let it rise for at least 30 minutes. If you have time, let it rise for an hour or two until it is puffy and doubled in bulk (it will "grow" a lot).
Once that is complete, punch down the dough gently, then divide it into 12 equal portions (I received 20 pieces from my batch).
On a prepared cookie sheet, roll a portion into a ball.
On a cookie sheet, arrange all the pieces of dough.
Form a circle-shaped bun by gently pressing each piece down with your fingertips; I also made sure it wasn't too tall (make sure it's about the same height as other regular bread).
Make sure buns are golden brown after 15 minutes; do not overbake.
After it has cooled, let it dry on a rack.
Stir condensed milk and vanilla bean paste until creamy and smooth. Meanwhile, prepare the buttercream.
The buttercream should be spread on top of the buns once they are cool.
Have fun serving and enjoying