Preheat the oven to 350F/177C convection (325F/163C conventional) and grease and line four round 8 inch cake pans with baking spray and parchment paper (can do two at a time if you only have two pans)

In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt until they’re evenly distributed.

Place the butter and sugar in a large bowl or the bowl of a stand mixer and use an electric mixer to beat them at full speed for 1-2 minutes. Add the oil and beat for another 1-2 minutes at full speed.

Add in the eggs, one at a time, beating for at least 30 seconds between each one, until it’s incorporated.

Add the vanilla and sour cream and mix just until it’s combined.

Add half of the flour mixture, turn the mixer the low speed and stream in the milk. Add the other half of the flour mixture and increase the speed to medium for another 30-60 seconds, just until there are only a few streaks of flour left.

Use a rubber spatula to scrape the bottom and sides of the bowl and give it one more good mix by hand.

Distribute the batter evenly among the prepared pans, about 11oz in each pan.

Flip onto a tea towel or a cooling rack and allow to cool to room temperature before frosting.

Place the butter in a medium sauce pan over medium heat and swirl around just until it's almost melted.

Add the sugar and cocoa powder and whisk until they're smooth and well combined.

Stream in the milk as you continue whisking and then stir intermittently until the mixture comes to a boil.

Remove from the heat and whisk another few seconds. Pour the mixture into a heat safe bowl or jar, cover and chill until it's completely cooled.

Place the butter in a large bowl or the bowl of a stand mixer and use the whisk attachment to beat at full speed for 2-3 minutes. Make sure to scrape the edge of the bowl every so often.

Mix until the frosting is well combined with the marshmallows. The marshmallows will warm up your frosting and it may be too soft so place the bowl of frosting in the fridge for about 30-60 minutes, mixing at least halfway through.

Once everything has cooled and the frosting is chilled, place the first layer of cake onto a flat surface and poke some holes into the top half of your cake. I like to use a kabob skewer or a cocktail garnish pick for this but any type of thin skewer will work.

Spread a thin layer of the chocolate fudge sauce n top of the cake, trying to make it go into the holes.

Spread a layer of frosting on top of the fudge sauce and then pipe a border of frosting around the edge of the cake. I do this border fairly short (in height) because we’re just doing a thin layer of fudge sauce in the center but make sure the border is secure and there are no disruptions or the fudge sauce will run out.

Fill the border in with the chocolate fudge sauce and freeze the cake for 15 minutes or so.

Place the next layer of cake on top and repeat until the last layer of cake. I do the holes with the fudge sauce on top too and then freeze it for 30-60 minutes to make sure it’s sturdy before frosting the outside of the cake.

Use the rest of the frosting to lightly frost the cake (if you want to frost the cake fully, you’ll need at least another half batch of frosting and if you want to decorate the cake, you’ll need another full batch but I recommend skipping the marshmallows in the frosting for decorating).

Place a bunch of marshmallows on top of the cake and then drizzle with the chocolate fudge sauce.