1. Heat oven to 350°F.
2. Beat butter and brown sugar in large bowl until well blended. Add eggs, vanilla and instant coffee, if desired; 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.
3. 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.
4. 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.
1/2 cup (1 stick) butter or margarine
2/3 cup HERSHEY'S Cocoa
1/4 cup sugar
1 can (14 oz.) sweetened condensed milk (not evaporated milk)
1-1/2 teaspoons vanilla extract
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.