Line a baking sheet with foil. Butter the foil; set aside.
In a heavy 10-inch skillet combine walnuts, sugar, and butter. Cook over medium-high heat, shaking skillet occasionally until sugar begins to melt. Do not stir. Reduce heat to low and cook until sugar is golden brown, stirring occasionally. Remove from heat. Pour onto prepared baking sheet. Cool completely. Break into clusters. Store in airtight container in a cool, dark place for up to 1 week. Makes about 8 servings.