Preheat oven to 350°F (175°C). Grease a 9x13-inch pan with plenty of unsalted butter and a bit of flour. Pour the flour out, making sure it coats the pan. Set the pan aside.

Grab a large bowl and sift together cake flour, baking powder, baking soda, and salt. I'm usually team no sift, but with cocoa, I find it lumpy more often than not. So let's just get it done. Set the bowl of dry ingredients aside.

Pour the hot coffee into a small bowl and whisk in the cocoa powder until smooth. Set aside.

In a large bowl, whisk together the granulated sugar, dark brown sugar, oil, eggs, extra yolk, and vanilla until smooth.

Add ⅓ dry mixture, then ½ of the buttermilk, then the dry mixture again, and then the remaining buttermilk, ending with the dry ingredients. Mix until just combined. Do not overmix!

Fold in the sour cream and the coffee/cocoa mixture. Whisk the batter until it's smooth, then stop mixing. The batter will be slightly loose, but that's totally normal.

Pour into the pan and bake at 350F for 25–30 min, or until a toothpick comes out with a few moist crumbs. Cool completely.

Gently melt marshmallows with 1/4 cup heavy cream in a saucepan or microwave; stir until smooth. Add 1/2 tablespoon vanilla. Cool slightly.

Whip 1 cup heavy cream with powdered sugar to medium peaks. Gently fold in slightly cooled, melted marshmallow then continue whipping until you get stiff peaks.