Start by preheating your oven to 350ºF. While it’s warming up, grab your 9×13-inch cake pan and spray it with nonstick cooking spray. This step ensures your cake releases easily once baked.
In a medium bowl, whisk together the dry ingredients: all-purpose flour, graham cracker crumbs, baking powder, kosher salt, and ground cinnamon. Make sure there are no lumps; this will ensure an even texture in your cake.
In the bowl of a stand mixer fitted with the paddle attachment, beat together the softened butter and sugar until it’s light and fluffy. This process will take about 3 to 5 minutes. You want it to be pale in color and well combined.
Add in the eggs and vanilla extract. Mix these in until everything is well combined. The mixture should be smooth and creamy.
With the mixer on low speed, gradually add the flour mixture in three parts, alternating with the whole milk. Start and end with the flour mixture to keep the batter consistent. Mix just until combined after each addition.
Pour the batter into your prepared cake pan. Smooth the top with an offset spatula for an even bake. Bake for 30 to 35 minutes. Check for doneness by inserting a toothpick; it should come out clean or with a few moist crumbs attached.
Once baked, cool the cake completely in the pan on a wire rack. It’s essential to let it cool completely before adding the frosting to prevent it from melting off.
For the frosting, in the stand mixer, beat the softened butter on medium-high speed until it’s smooth. This process should take about 3 to 4 minutes. Make sure to scrape down the sides of the bowl as needed.
Add the marshmallow creme and kosher salt into the butter, and continue beating until just combined.
Slowly add the powdered sugar and cocoa powder, mixing on low until it’s all incorporated. Then, gradually pour in the heavy whipping cream. Mix until just combined, then turn the mixer to high and beat until the mixture becomes light and fluffy, about 2 to 3 minutes.
Once the cake is completely cooled, spread the frosting generously over the top. Use your offset spatula to create swirls or smooth it out, depending on your preference.