1. Combine brown sugar, milk and butter in a medium saucepan. Stir constantly over medium heat until mixture reaches 235 (soft ball stage) on a candy thermometer. Remove from heat and let cool to room temperature. 2. Fit steel blade in work bowl of food processor and process until mixture is light brown, about 20 seconds. 3. Add vanilla and peanuts, then mix with 3-4 quick on/off motions. 4. Pour into buttered 8-inch square baking dish. Let cool completely, then cut into 1 x 2-inch pieces.