Heat oven to 350. Cool mashed Potatoes to room temperature.
On floured work surface, roll a biscuit out to a 6 inch circle. Place 2 heaping tablespoons of potatoes in center of bicuit. Draw up dough and pinch edges together using a fork, to cover filling. Repeat to form 8 pagachi.
Place pagachi on lightly greased baking sheet. Prick each with a fork a few times and brush lightly with egg whites.
Bake for 20 minutes.
Meanwhile, place butter in a small skillet over medium heat and lightly brown.
Remove pagachi from oven and brush with the browned butter. Serve warm.