Preheat oven to 350 degrees.

In a mixing bowl, blend butter until creamy with an electric mixer. Add sugar and mix until blended. 

Add eggs and mix until just blended.  Stir in vanilla.

Mix water and buttermilk together. (wet ingredients) 

In a separate bowl, mix flour, cocoa powder, and baking soda together. (dry ingredients)

Alternate adding dry ingredients and wet ingredients with contents in mixing bowl until well blended. Beat at low speed.

Pour batter into a 13 x 9 inch baking dish (ad) or pan that has been greased and floured. Bake at 350 degrees for 30 minutes or until toothpick comes back clean when inserted in middle of cake.

Cool cake while making frosting.

Melt butter in a saucepan on medium heat.

Stir in half & half and bring to a boil.

Take off heat and stir in peanut butter until creamy.

Pour into a mixing bowl and add powdered sugar gradually while mixing at medium-low speed.

Add vanilla and stir well.

Pour icing over sheet cake.

Sprinkle salted peanuts and chopped peanut butter cups over top of cake.