Chocolate Peanut Butter Sheet Cake

This was a combination of two recipes, one from Our Best Bites, and the other from Allrecipes. As good as it was when we made it, it was quite heavenly the next day, so I'd recommend letting it sit overnight. (And I was lucky to get a taste the next day because a certain boy in our house found the tupperware of a few leftover pieces of cake in the morning before we'd gotten up, and nearly ate it all before we found him. I don't know what to do with him.)
The cake is very moist and fluffy, and the peanut butter frosting, which isn't too sweet, makes the combination taste like Reese's Peanut Butter Cups.

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CAKE
1 cup water
1 cup (2 sticks) butter
1/2 cup unsweetened cocoa
2 cups sugar
1 3/4 cups flour, spooned lightly into measuring cups and leveled with a knife
1 teaspoon baking soda
1/2 teaspoon table salt
3 eggs
3/4 cup buttermilk or sour cream
1 teaspoon vanilla extract

FROSTING
1/2 cup butter, softened
1 cup creamy peanut butter
2 cups powdered sugar
3 Tbsp milk
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Preheat oven to 350. Grease a 9×13″ pan and set it aside.

 In a medium saucepan, combine the water, butter, and cocoa powder. Over medium heat, stir the mixture frequently until it comes to a boil. Boil for 1 minute, then remove from heat.

 In a large bowl (or the bowl of your stand mixer), add the flour, sugar, baking soda, and salt and whisk together to combine.

Add the eggs, vanilla, and buttermilk (or sour cream) and beat until blended.

Add the cocoa mixture and beat until just combined. Pour into the prepared pan.

Bake for 30-35 minutes or until a pick inserted into the center comes out clean. Allow to cool completely.

For the frosting, place the butter and peanut butter into a medium bowl, and beat with an electric mixer.

Gradually mix in the sugar, and when it starts to get thick, incorporate milk one tablespoon at a time until all of the sugar is mixed in and the frosting is thick and spreadable.

Beat for at least 3 minutes for it to get good and fluffy.

Spread it on the cake. It'll be nice and thick.