Using a countertop mixer fitted with dough hook attachment, add all-purpose flour, potato flour, yeast, sugar, salt, milk, egg yolk and softened butter. (Note: The remaining 3 Tablespoons of melted butter will be brushed on rolls later.)
Mix on low speed for 2 minutes. Increase speed to medium and continue mixing for 3-4 more minutes.
Transfer dough into a large oiled bowl; cover with plastic wrap. Place in a warm (85°F) location for 90 minutes, or until dough has almost doubled in size. (Tip: I typically let my dough rise in the oven with the oven light on. The light is on, but the oven itself is off.)
Transfer the dough to a lightly floured surface. Divide dough into 2 equal portions.
Working with one piece of dough at a time, press/roll it into a 6”x9” rectangle.
Brush the top of the dough lightly with melted butter. (Note: You won’t use all of the butter at this step.)
Cut the dough in half lengthwise to create (2) 3”x9” rectangles. Gently lift a long edge of the dough and fold lengthwise to within ½” of the other long edge. (The bottom edge of the dough should stick out ½” further than the top.) Repeat process with other rectangle.
Cut each rectangle crosswise into (3) 3” pieces.
Lightly spray a 9”x13” baking pan with nonstick cooking spray.
Flip rolls over and place folded side down in prepared pan.
Repeat process with the remaining piece of dough, creating a total of 12 rolls.
Spray tops of dough with nonstick cooking spray and cover lightly with plastic wrap. Place pan in a warm location for 45-60 minutes, or until dough has risen but not quite doubled.
Preheat oven to 350°F. (Note: If letting dough rise in oven, make sure to remove pan before preheating oven!)
Bake rolls for 20-22 minutes, or until tops are golden brown in color.
Remove rolls from oven and brush with remaining melted butter.