This recipe comes from a friend of mine whose Mother worked it the local schoolhouse cafeteria. This is the original peanut butter bars that were served and they are delicious, just as I remember them as a child.
Put the peanut butter and butter in a saucepan over low heat to melt, do not let it boil, just warm and melt. Remove from heat and stir in the sugar, eggs and vanilla. Add the flour and stir to combine well. Spread evenly into a 13x9" sheet cake pan. Bake at 350 degrees for 25 minutes. Cool completely and frost.
Whisk together the last 4 ingredients and spread over bars.