Combine water and oats-cool to room temperature. Set aside. Cream butter, sugars and vanilla. Blend in oatmeal mixture. Beat in eggs; combine dry ingredients, add to creamed mixture. Beat 1 minute at medium speed. Pour into greased & floured 9x13 pan. Chop peanut butter cups & sprinkle on top of batter. DO NOT STIR!!!!! Bake at 350 degrees 40-45 minutes. Remove from oven & cool.
Frosting: melt butter with chocolate; in small pan stir constantly, over very low heat, until melted. Pour into small mixing bowl. Add remaining ingredients-(powdered sugar, vanilla, salt & milk)Beat until well blended. Chill to spreading consistency-10-15 minutes. Frost cake. If there are leftover Reese Peanut Butter Cups, Slice & drop on top of icing. Yummy!!!!!