Heat oven to 350°F Beat butter and brown sugar in large bowl until well blended.
Add eggs, vanilla and instant coffee and beat until fluffy.
Stir together oats, flour, baking soda, salt and 1 cup walnuts; gradually stir into butter mixture with spoon (Dough will be thick).
Set aside 2 cups dough.
Press remaining dough evenly onto bottom of ungreased 15-1/2x10-1/2x1-inch jelly-roll pan.
Prepare CHOCOLATE FILLING; spread evenly over dough.
Crumble reserved dough evenly over filling. Sprinkle with remaining 1/2 cup walnuts. Bake 25 minutes or until top is golden brown (chocolate will be soft).
Cool completely in pan on wire rack; cut into bars.
About 4 dozen bars.
CHOCOLATE FILLING: Melt butter in medium saucepan over low heat. Stir in cocoa and sugar. Add sweetened condensed milk; cook, stirring constantly, until smooth and thick. Remove from heat. Stir in vanilla.