Heat oven to 450°F and grease a baking sheet or use parchment paper.
In a big bowl, mix flour, potato starch, and potato flour; set aside.
Mix milk, eggs, and yeast; stir well.
In a small bowl, add 2 1/4 teaspoons of sugar to yeast mixture; wait for 5-10 minutes until it gets foamy (bubbles appear).
In another bowl, beat shortening until smooth, add softened butter, then beat until it mixes well.
Add 2 tablespoons of sugar and salt to the butter/shortening mixture.
In a big bowl, mix yeast mixture with butter/shortening mixture.
Add flour mixture and stir until it mixes just right. The dough will be sticky.
Knead the dough with your hands or use an electric mixer.
Cover with plastic wrap and let it rise for at least 30 minutes (or longer for it to get bigger).
Gently push the dough down and divide it into 12 equal parts.
Roll each part into a ball and put them on the baking sheet.
Gently press down on each ball to make them into circle-shaped buns.
Bake for about 15 minutes or until they turn golden brown; don't let them burn.
Let them cool on a rack.
For the buttercream, mix condensed milk and vanilla bean paste until it's smooth.
Once the buns are cool, spread 2-3 tablespoons of buttercream on each.
Serve, enjoy, and share!