Preheat oven to 180 degrees C. Grease a 20x30cm baking dish.
In a large bowl cream together the butter, brown sugar, egg yolk and vanilla until smooth. Combine the flour and salt then stir into the creamed mixture. Spread evenly into the prepared dish.
Bake for 15 minutes in the preheated oven. Remove from the oven and place chocolate over the top in a single layer. Let stand until chocolate is soft enough to spread. Spread the chocolate out smooth and sprinkle with walnuts and ground coffee. Allow the bars to cool completely before cutting into squares.