Line a baking sheet with foil, and butter the foil.
In a heavy 8-inch skillet, combine almonds, sugar, and butter. Cook over medium heat for about 9 minutes, stirring constantly, until sugar melts and turns a rich brown color. Remove from heat and stir in vanilla.
Pour onto baking sheet. Cool completely before breaking into clusters. Keep in an airtight container at room temperature for about 2 weeks.