Chocolate Peanut Butter Sheet Cake with a Marshmallow Peanut Butter Frosting

Bart made this cake for us today. It’s a yummy, moist and tender cake that is easy to serve. The hint of cinnamon and touch of peanut butter are a welcome surprise to what looks like a chocolate cake.

For the Chocolate Peanut Butter Sheet Cake:
2 cups sugar
1 ½ cups all-purpose flour
1/3 cup cocoa powder (preferably natural cocoa powder)
2 teaspoons ground cinnamon
1 teaspoon baking soda
½ teaspoon salt
¾ cup creamy peanut butter
2 large eggs, at room temperature
1 ½ cups buttermilk (regular or low-fat but not nonfat)
12 tablespoons unsalted butter, melted and cooled
2 teaspoons vanilla extract

For the Marshmallow Peanut Butter Frosting:
¾ cup creamy standard peanut butter
1 cup Marshmallow Fluff or Marshmallow Crème
¼ cup milk (preferable low-fat or nonfat)
3 to 4 cups confectioners’ sugar

Preheat the oven to 350°F. Grease and flour (or spray with Baker’s Joy) a 13 x 17-inch jelly-roll pan.

Whisk the sugar, flour, cocoa powder, cinnamon, baking soda, and salt in a medium bowl; set aside.

Use an electric mixer to beat the peanut butter and eggs until creamy, about 1 minute. Add the buttermilk, melted butter, and vanilla; beat until smooth. Remove the beaters and stir in the prepared flour mixture with a wooden spoon (or KAF dough whisk) until smooth but perhaps still a little grainy—there should be no white streaks of flour visible but overbeating will stretch and align the glutens and render the cake less tender. Pour this batter into the prepared jelly-roll pan.

Bake until a toothpick inserted in the center of the cake comes out with a few moist crumbs attached, about 30 minutes. Place the pan on a wire rack and cool completely, about 2 hours.

To make the frosting, use an electric mixer on medium speed to beat the peanut butter and Marshmallow Fluff in a large bowl until smooth, about 1 minute. Beat in the milk, then scrape down the sides of the bowl and beat in the confectioners’ sugar in 1/3-cup increments until a smooth, creamy frosting forms, one that can hold its shape when scooped onto a knife or icing spatula.

Place small dollops of the frosting all over the cake, then use an icing knife or an offset spatula to smooth the frosting evenly over the cake.