Preheat the oven to 350 degrees F. Grease and line three 6 inch cake pans with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, graham cracker crumbs, sugar, baking powder, and salt.

Cut up the butter into small cubes and add to the dry ingredients. Mix on low speed until it resembles wet sand.

Add the oil, milk, egg, and vanilla, and mix until just combined.

Divide the cake batter between the prepared pans.

Bake for 22-25 minutes or until a toothpick inserted comes out clean. Immediately turn the cakes out onto a wire rack to cool completely.

Once cool, wrap the cakes in plastic wrap and freeze until ready to decorate. (Frozen cake layers will be much easier to frost!)

In a small saucepan, place the water, sugar, and light corn syrup over low heat, stirring constantly until the sugar dissolves. Turn the heat up to medium, and bring to a boil, or until it reaches 240 degrees F.

While the sugar syrup is heating, add the egg white to a medium bowl and use an electric mixer to beat it to soft peaks.

Once the syrup has reached 240 degrees F, immediately drizzle it into the whipped egg white while mixing on low speed. Turn the mixer up to high speed until thick and glossy.

Lastly, mix in the vanilla extract.

Cover the bowl and set aside.

In a small heat-proof bowl, add the chocolate and place it over a pot of simmering water. Stir often until fully melted. Remove the bowl from heat and set aside.

Place the egg yolks and 24g (2 tbsp) of sugar in the bowl of a stand mixer fitted with a whisk attachment. Whisk on high speed until thick and foamy (about 5 minutes) while you heat the sugar and water.

In a small saucepan, place the remaining 100g (½ cup) of granulated sugar and water over low heat, stirring constantly until the sugar dissolves. Increase the heat to medium and bring to a boil, or until it reaches 240 degrees F.

While the egg yolks are still whisking on medium speed, slowly drizzle the sugar syrup into the bowl. Continue mixing until the mixture has cooled down to room temperature.

Add the butter to the mixture a few cubes at a time, allowing them to fully incorporate before adding more.

Lastly, add the melted chocolate and vanilla. Continue mixing on low speed until the buttercream is completely smooth.

Place the first cake layer on a cake stand. Use a 3 tablespoon cookie scoop to add 1 scoop of buttercream and spread into an even layer. Then, add a few scoops of marshmallow fluff and spread into an even layer. Repeat with the next cake layer.

Add the last cake layer and top with a few more scoops of buttercream. Use an offset spatula to smooth out the top, and spread the excess all over the sides. Continue decorating the cake as desired.

Top the cake with marshmallows, and enjoy!