Preheat the oven to 325, and grease a quarter-sheet pan. Line the bottom of the pan with parchment paper for easy removal, and set aside.

In a large bowl, whisk together the brown sugar, melted butter, and canola oil until combined, and then mix in the egg and vanilla extract. In a small bowl, stir together the flour, salt, baking powder, and baking soda. Stir the flour mixture and the buttermilk into the egg mixture in alternating additions, starting and ending with the flour mixture. You want to mix well between each addition until everything is just combined and you have a smooth batter.

Spread the batter evenly into your prepared cake pan, and bake for 12-15 minutes, or until a cake tester comes out with just a few crumbs clinging to it. Optionally, you can use an instant read thermometer that should read 200 degrees when the cake is done. Cool the cake completely before adding the topping.

In a microwave safe bowl, break up your chocolate into chunks and microwave the mixture for 30 seconds. Stir, and repeat microwaving in 30 second increments until the chocolate is melted. Pour the melted chocolate on top of the cooled cake, and use the back of a spoon or an off-set spatula to spread it into an even thin layer over the cake. Let this layer of chocolate fully set before you add the marshmallow topping.

Place a large heat-proof bowl, or the metal bowl of your stand mixer, over a pot of simmering water (the water should not touch the bowl). In this bowl, stir together the sugar, egg whites, corn syrup, and salt. Stir this mixture over the heat for 5-10 minutes, or until the sugar has dissolved, and then remove it from the heat.

Using an electric mixer or your stand mixer, whisk until the mixture has thickened dramatically, and holds stiff peaks, which should take about 10 minutes. Add in the vanilla extract and stir to combine. Use a spoon to gently pile the frosting on top of your cake, and gently make swirls with small peaks if desired.

To toast the meringue, the easiest option is to use a kitchen torch and toast the top of the frosting to your desired brown/black. Optionally, you can place the cake under the broiler on high in your oven for 3-5 minutes. Slice and serve!