Three-Chip English Toffee

Butter a 15x10x1-in. pan with 1/2 teaspoon butter. In a heavy saucepan over medium-low heat, bring sugar and remaining butter to a boil, stirring constantly. Cover and cook for 2-3 minutes., Uncover; add almonds. Cook and stir with a clean spoon until a candy thermometer reads 300° (hard-crack stage) and mixture is golden brown., Pour into prepared pan (do not scrape sides of saucepan). Surface will be buttery. Cool for 1-2 minutes. Sprinkle with milk chocolate chips. Let stand for 1-2 minutes; spread chocolate over the top. Sprinkle with walnuts; press down gently with the back of a spoon. Chill for 10 minutes., In a microwave, melt semisweet chips; stir until smooth. Drizzle over walnuts. Refrigerate for 10 minutes. Melt vanilla chips and shortening; stir until smooth. Drizzle over walnuts. Cover and refrigerate for 1-2 hours. Break into pieces.