Preheat oven to 350ºF degrees. Line three 6-inch round cake pans with parchment paper and spray with nonstick cooking spray.

In a large mixing bowl, whisk the flour, baking soda, baking powder, cinnamon, salt, crushed graham crackers, and both sugars together until well combined. Add the oil, yogurt, and milk and continue stirring with a whisk. Stir in the egg, followed by the vanilla.

Pour batter into prepared baking pans and bake in the oven for 30-33 minutes, or until a toothpick inserted in the center comes out clean. Allow cakes to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Preheat the broiler. Evenly scatter the marshmallows on a baking sheet. Place the sheet on the middle rack in the oven and bake for 1 to 2 minutes. Keep a close eye on them, so that they don’t scorch since it can happen pretty quickly. When the marshmallows are golden brown, remove the pan from the oven; set aside to cool slightly.

In a stand mixer fitted with the whisk attachment, beat the heavy cream until stiff peaks form. Transfer whipped cream to a separate bowl, then return bowl to mixer. Switch to the paddle attachment.

To the bowl, add the cream cheese, powdered sugar, and vanilla. Beat on high until smooth. Add in the toasted marshmallows, and beat on high until marshmallows have broken up and mixture is smooth.

Add half of the whipped cream to the bowl and continue beating until combined. Remove the bowl from the mixer and fold in the rest of the whipped cream. Transfer filling to a pastry bag or a gallon-sized ziploc bag with the tip of the corner cut off; refrigerate until cake assembly.

Place milk chocolate in a microwave-safe bowl, and heat in the microwave until smooth and melted. Set aside to cool slightly while you work on the rest of the frosting.

Combine the egg whites, sugar and salt in the bowl of a stand mixer, and place the bowl over a simmering pot of water. Whisking constantly, heat mixture until sugar is dissolved and the temperature registers 150-160°F on a candy thermometer, about 3 minutes.

Secure whisk attachment onto stand mixer, and beat the egg white mixture on high speed until cooled and fluffy. Add the butter, one piece at a time, until smooth and creamy. If the mixture looks curdled, that’s totally normal--just keep beating until it becomes smooth and satiny.*

Stir in the vanilla. Pour in the melted chocolate and mix until combined, scraping down the sides of the bowl to ensure that there's no melted chocolate stuck to the bowl. Increase speed to medium-high and beat until light and fluffy, about 30 seconds.

Transfer about a cup of the frosting to a piping bag, fitted with a round tip.

Place a cake layer on a cake plate or pedestal. Pipe a border of buttercream along the perimeter of the cake to prevent filling from peeking out of the sides. Using a spiral motion from the outside in, pipe a third of the filling on top and inside the buttercream border. Repeat steps with the remaining cake layers and filling, ending with a layer of filling on top. Chill cake in the freezer for 1 hour.

Once chilled and solid, frost the cake with the buttercream--this will act as the crumb coat. Chill cake in the freezer for another 30 minutes to set the buttercream. Remove the cake from the freezer and frost it with a final layer of buttercream. Return the cake to the freezer while you make the ganache.

Make the ganache by placing the chocolate in a medium bowl. Pour the heavy cream into a glass measuring cup, and heat in the microwave until it starts to boil. Pour hot cream over the chocolate, cover with plastic wrap, and allow it to sit for a few minutes. Remove the plastic wrap and stir until smooth.

Using a small spoon, carefully ladle the ganache along the edges of the cake, allowing the ganache to drip down the sides.** Once the sides are done, pour a few tablespoons of the ganache into the center of the cake, and use an offset spatula to fill in the top of the cake.

Decorate the cake with homemade s'mores, toasted marshmallows, chocolate, and leftover chocolate buttercream. Slice, serve, and enjoy!

Cake will keep covered in the refrigerator for up to 5 days.