Cream butter/margarine, sugar and peanut butter. Add beaten egg. Mix until creamy.
Mix baking soda into flour and add into creamed mixture alternatly with milk. Blend until it looks a little fluffy. Usually about a minute on medium speed with a hand mixer. Bake in 350 degrees oven until done. Usually about 45 min or so.
For the frosting (make while cake is cooling). Combine all ingredients and mix on medium speed with hand mixer until creamy. If too thin, you can add a bit more powered sugar or a bit more peanut butter or cream cheese depending on the flavor you prefer.
Frost cake when cooled. You can put this in 9 inch round pans for a layer cake or in a 9 x 13 in pan. More frosting would need to be made for a layer cake or you can always use something else for the filling.