Preheat oven to 190 C/ Gas 5. Grease a 23cm rectangular baking tray.
Beat butter and brown sugar together. Add egg yolk, sifted flour and vanilla. The mixture will be stiff.
Spread the mixture evenly onto the greased baking tray. Bake for 12-15 minutes until golden brown.
While still warm, sprinkle the chocolate chips over the top of the biscuit layer. They will begin to melt; use a palette knife to spread the chocolate evenly over the biscuit layer. Add chopped walnuts. Leave to cool, then cut into fingers or squares.