Melt butter in small saucepan over low heat; stir in cocoa. Pour mixture into small mixer bowl; gradually beat in sugar. Add eggs ; beat well.
Combine oats, flour, baking powder and salt; add to chocolate mixture, blending well. Stir in vanilla and walnuts.
Spread in greased 9-inch square pan. Bake at 350 for 30 to 35 minutes or until cookie begins to pull away from edges of pan. Cool slightly; cut into bars. Cool completely.
