Place potatoes into a pot with enough water to cover. Bring to a boil, and cook until tender. Drain, and set aside.

Heat 1/4 cup butter in a large skillet over medium heat. Stir in onions, and cook until soft and translucent.

Mash the potatoes until smooth in a large bowl. Mix in onion and butter mixture, crumbled bouillon, and salt and pepper to taste.

Preheat oven to 375°F (190°C). Grease a baking sheet.

Layer 3 sheets of phyllo dough one on top of the other, and cut this triple-layered sheet in half. Drop one spoonful of potato mixture into each phyllo piece. Roll dough around mixture, tucking ends under. Repeat with remaining sheets to make 12 knishes. Brush with remaining butter, and place on baking sheet.