Preheat oven to 350 degrees. Beat together butter and brown sugar until creamy. Add eggs, vanilla and instant coffee. Beat well. Stir together oats, flour, baking soda, salt and 1 cup walnuts. Gradually add oat mixture to butter mixture, beating well until blended (batter will be stiff, stir by hand if necessary at this point). Remove 2 cups of the dough and set aside. Press remaining dough into bottom of 15 1/2 by 10 1/2 inch jelly roll pan.
For filling: In medium saucepan over low heat, melt butter. Add chocolate. Cook, stirring, until smooth. Stir in sugar and sweetened condensed milk. Cook, stirring constantly, until mixture thickens and sugar is dissolved. Remove from heat. Stir in vanilla. Spread chocolate filling over crust in jelly roll pan. Sprinkle reserved dough over filling. Sprinkle remaining 1/2 cup walnuts over all. Bake 25 minutes or until top is golden brown. Cool completely in pan on wire rack. Serve cut into bars