Heat oven to 350. Beat butter and brown sugar until well blended. Add eggs, vanilla and instant coffee(if using), beat 'til fluffy.
Stir together oats, flour, baking soda, salt and 1 cup walnuts. Gradually stir into butter mixture, will be thick.
Set aside 2 cups of dough. Press remaining dough into a 15-1/2x10-1/2 greased jelly roll pan.
Prepare filling by melting butter in medium sauce pan over medium low heat. Stir in cocoa and sugar. Add condensed milk and continuously stir until smooth and thick. Remove from heat and stir in vanilla. Spread over dough evenly.
Crumble reserved dough over top and sprinkle with 1/2 cup walnuts. Bake for 25 mins. or until golden brown (chocolate will be soft). Allow to cool before cutting.