Peanut Butter Sheet Cake

Step 1
Preheat the oven to 350 degrees F (175 degrees C). Grease a 10x15x1 inch jellyroll pan.

Step 2
In a large bowl, stir together the flour, 2 cups sugar, baking soda and salt. Set aside. Combine the water and 3/4 cup of butter in a saucepan, and bring to a boil. Remove from the heat and stir in 1/2 cup peanut butter and vegetable oil until well blended. Stir this mixture into the dry ingredients. Combine the eggs, buttermilk and vanilla; stir into the peanut butter mixture until well blended. Spread the batter evenly in the prepared pan.

Step 3
Bake for 18 to 26 minutes in the preheated oven, or until a toothpick inserted near the center comes out clean.

Step 4
While the cake bakes, place 2/3 cup sugar, evaporated milk, and butter in a saucepan. Bring to a boil, stirring constantly. Cook stirring for 2 minutes. Remove from heat and stir in the peanut butter, marshmallows and vanilla until marshmallows are melted and the mixture is smooth.

Step 5
Spoon the frosting over the warm cake and spread in an even layer. Allow to cool before cutting and serving.