Cream 1/2 cup peanut butter, butter and sugars together. Add egg and vanilla. Add all dry ingredients, blending well. Spread dough into a buttered 9x13-inch pan and bake at 350 degrees F. for 20-25 minutes.
Remove from oven and immediately sprinkle chocolate chips on top. Let stand 5 minutes, then spread the chocolate chips to cover the bars.
Combine powdered sugar, peanut butter and milk, adding enough milk to make a thin consistency. Drizzle over chocolate layer. Sprinkle with chopped nuts.
Cut into bars for serving.