Preheat the oven to 350°F. Lightly coat a 9x13 cake pan with cooking spray. Set aside.
In a medium mixing bowl combine the flour, sugar, brown sugar, baking soda and salt. Whisk well. Set aside.
In a medium saucepan bring 3/4 cup of the water and butter to a boil. As soon as the mixture boils, reduce heat to low and stir in peanut butter and vegetable oil. When mixture is smooth, set aside and remove from heat, cool slightly before adding to other wet ingredients.
In a large mixing bowl with electric mixer or a stand mixer fitted with a paddle attachment, combine eggs, powdered buttermilk and remaining 1/2 cup water. Beat until smooth. Add cooled peanut butter mixture. Mix until cake batter is smooth.
Gradually add dry ingredients until fully combined. Batter will be light and slightly soupy. This is normal. Pour into greased cake pan.
Bake for 30 minutes or until top is a light golden brown and the center passes the toothpick test. Remove from the oven and frost immediately.
About 5 minutes before the cake is scheduled to be removed from the oven start to make the frosting.
In a medium saucepan, combine the sugar, evaporated milk and butter and heat over medium-high, stirring continuously.
When sugar is dissolved, add the peanut butter, marshmallows and vanilla. Stir until smooth and fully incorporated over very low heat.
Pour warm frosting mixture over the hot Peanut Butter Cake and spread around to cover fully. Sprinkle with honey roasted peanuts immediately while frosting is still tacky.
Allow to cool completely before serving.
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