Preheat the oven to 350°F and line a 9 x 13-inch baking pan with parchment paper.
Add the butter, brown sugar, vanilla and egg yolks to a large bowl and beat on the lowest speed until smooth and combined.
Stir in the flour, salt and 1 cup of the toffee bits, the mixture will be very thick and crumbly.
Dump it into the pan and press it out evenly with wet fingers.
Bake 25 minutes. Remove from oven and sprinkle the chocolate chips over the top of the crust. Let them melt about 5 minutes then spread over the top with a rubber spatula.
Sprinkle the remaining toffee bits over the chocolate. Let cool completely then refrigerate about 20 minutes before cutting to help the chocolate set up.
Lift using the parchment paper to a cutting board and cut into squares with a large chef's knife. Store tightly covered up to a week.