Preheat oven to 350 degrees.

Cream together the butter and brown sugar.

Add the egg yolk and vanilla; stir to combine.

Mix in the flour.

Pat the dough into an ungreased 15 x 10″ cookie sheet. (I have to say, this dough was quite sticky, however I found that if I wet my hands a little it made spreading the dough a lot easier!)

Bake for 18-20 minutes; until lightly brown around the edges.

Remove from oven and immediately pour on the chocolate chips.

Wait a few minutes then evenly spread over the bars.

Sprinkle on the chopped nuts.

Slice, serve and enjoy!