Place the butter and brown sugar in the bowl of a stand mixer. Beat until light and fluffy, about 2 minutes.
Add the egg yolk and vanilla, and beat until combined.
Add the flour and salt, and beat just until combined.
Press into a 13x9-inch pan. Bake for 22–24 minutes, or just until lightly golden brown on the edges.
Remove the pan from the oven and sprinkle the chocolate chips evenly over the top. Let side for 5 minutes to melt the chocolate, then spread the chocolate evenly over the cookie base.
Sprinkle with nuts and let cool until the chocolate is mostly set, but still slightly glossy on top. Cut into squares.
Notes
Cutting the bars while the chocolate is still slightly glossy on top ensures the chocolate stays put (and doesn't crumble off) when you cut the bars. Topping ideas (in place of the nuts): sprinkles, chopped pretzels, chopped toffee bars such as Heath, crushed candy canes.You can store these in an airtight container on the countertop for up to two weeks. You can also freeze them for up to two months.If you remember a slightly different bar, some recipes call for ⅔ cup of chocolate chips and ½ cup of chopped nuts. The quantities in the recipe above are true to my grandma's recipe.