Heat oven to 350°F Line a 15 1/2 x 10 1/2 inch rimmed baking sheet with parchment paper; set aside.
In a large bowl, beat butter with brown sugar until fluffy. Beat in egg yolk and vanilla until creamy.
Stir in flour and 1 cup of the toffee bits until well combined. Press dough evenly into prepared baking sheet.
Bake for 18-20 minutes or until light brown; transfer pan to a cooling rack. Immediately sprinkle with chocolate chips over the hot cookie base. Let stand for 5 minutes or until chocolate is softened. Spread chocolate evenly over the base; sprinkle remaining toffee bits evenly over chocolate. Cool until chocolate is set, about 2-3 hours. Cut into 3-inch squares; cut each square diagonally to make 4 triangles.