In large bowl, stir together flour, brown sugar, cocoa and egg. Cut in 1 1/4 cups of the butter, using pastry blender (or pulling 2 table knives through ingredients in opposite directions), until crumbly. Stir in walnuts; reserve 3 cups of the crumb mixture. Press remaining mixture firmly in bottom of pan; sprinkle with 2 cups chocolate chips. Bake 15 minutes.