Set your oven to 350°F (175°C) and get your baking sheets ready with a layer of parchment. Trust me, don’t skip this step, it saves your cakes from sticking and makes cleanup way easier.

In a mixing bowl, whisk together both sugars, the oil, the whole egg and extra yolk, plus vanilla extract until the mixture is nicely combined and smooth. Then stir in just half of your buttermilk, this helps everything blend easily and gives the batter that rich, soft texture later on.

In another bowl, whisk your dry ingredients: the all-purpose flour, Dutch-process cocoa powder, baking soda, baking powder, salt, and that pinch of espresso powder if you're using it. Mix until everything looks evenly combined.

Combine the wet and dry ingredients by pouring and whisking softly. Incorporate the leftover buttermilk with gentle folds, taking care not to overwork the batter. A few lumps are totally fine, they’ll bake out beautifully.

Use a tablespoon or cookie scoop to portion the batter onto your prepared sheets, around 2 tablespoons per whoopie. Try to leave enough space between each one, since they’ll spread and puff up a bit while baking.

Slide them into the oven and bake for 10 to 11 minutes. You’ll see they’re ready once the middle springs back with a light touch. You can also check with a toothpick, you’re looking for moist crumbs, not raw batter.

Once they’re baked, transfer them to a wire rack and let them cool down completely. For the filling, beat the salted butter in a bowl until it’s completely smooth and fluffy. Slowly add in the powdered sugar and keep beating until everything looks light and well blended.

Next, add the marshmallow fluff and 1 tablespoon of heavy cream, then beat again until the filling is airy and spreadable. If it’s too thick, you can add a bit more cream to loosen it up.

Stir in the vanilla extract for that little extra flavor boost. Now, take half of your cooled cake rounds, spread or pipe a generous layer of filling on the flat side, and gently sandwich another cake on top. Enjoy!