Sheet-Pan Graham Cracker Cake

Preheat oven to 350°F. Coat a 13- x 9-inch baking pan with cooking spray; set aside. Whisk together flour, graham cracker crumbs, baking powder, salt, and cinnamon in a medium bowl until combined. Set aside.

Beat together sugar and butter with a stand mixer fitted with a paddle attachment on medium speed until light and fluffy, 3 to 4 minutes. Reduce mixer speed to low speed; add eggs one at a time, beating until yellow disappears after each addition. Scrape down sides of bowl; add vanilla and beat on low speed until combined, about 30 seconds.

With mixer on low speed, gradually add flour mixture in 3 additions, alternating with milk, beginning and ending with flour mixture, until just combined after each addition, stopping to scrape down sides of bowl as needed, about 3 minutes total. Transfer batter to prepared pan and spread into an even layer.

Bake in preheated oven until golden brown and a wooden pick inserted in center comes out with a few moist crumbs, 30 to 35 minutes. Let Cake cool to room temperature in pan on a wire rack, about 1 hour.

Beat butter with a stand mixer fitted with a paddle attachment on medium-high speed until smooth, about 1 minute. Add marshmallow creme and salt, and beat on medium-high speed until just combined, about 1 minute. Reduce mixer speed to low; gradually add powdered sugar and cocoa, beating until just combined after each additional, about 3 minutes total. With mixer on low speed, gradually add heavy cream; beat until just combined, about 1 minute. Scrape down sides and bottom of bowl, increase speed to high, and beat until fluffy, about 2 minutes. Spread Chocolate Marshmallow Buttercream evenly over top of cooled Graham Cracker Cake. Slice and serve.