Peanut Butter Sheet Cake

2 cups all-purpose flour
2 cups granulated sugar
1/2 teaspoon baking soda
1/4 teaspoon salt
1 cup water
3/4 cup butter or margarine, softened
1/2 cup peanut butter
1/4 cup vegetable oil
2 eggs
1/2 cup buttermilk
1 teaspoon vanilla extract

Frosting:
2/3 cup granulated sugar
1/3 cup evaporated milk
1 tablespoon butter or margarine
1/3 cup chunky peanut butter
1/3 cup miniature marshmallows
1/2 teaspoon vanilla extract

Heat the oven to 350 degrees F (175 degrees C). Grease a 10 x 15 x 1-inch jellyroll pan.

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 extract; stir into the peanut butter mixture until well blended. Spread the batter evenly in the prepared pan. Bake for 18 to 26 minutes in the preheated oven, or until a wooden pick inserted near the center comes out clean.

While the cake bakes, make the frosting. 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 extract until marshmallows are melted and the mixture is smooth. Spoon the frosting over the warm cake and spread in an even layer. Allow to cool before cutting and serving.