Follow these detailed instructions carefully to ensure your whoopie pie bites are flawless. We’ll break the process down into three manageable stages: making the cakes, whipping up the filling, and assembling the final treats.

Part 1: Making the Chocolate Cakes

1. Preheat and Prepare: Begin by preheating your oven to 375°F (190°C). Line two large baking sheets with parchment paper or silicone baking mats. This is a critical step that prevents the cakes from sticking and ensures easy cleanup.
2. Combine Dry Ingredients: In a medium-sized bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking soda, and salt. Whisking is important as it aerates the ingredients and ensures they are evenly distributed, which is key for a uniform rise and flavor. Set this bowl aside.
3. Cream Butter and Sugar: In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the softened unsalted butter and packed light brown sugar together on medium-high speed. Continue creaming for about 3-4 minutes, until the mixture is light, pale, and fluffy. This process incorporates air into the batter, which is essential for a tender cake.
4. Add Egg and Vanilla: Scrape down the sides of the bowl with a rubber spatula. Add the room temperature egg and vanilla extract to the creamed butter and sugar. Beat on medium speed until everything is well combined and the mixture is smooth, about another minute.
5. Alternate Wet and Dry Ingredients: Reduce the mixer speed to low. Add about one-third of the dry ingredient mixture to the wet ingredients and mix until just barely combined. Then, pour in about half of the buttermilk and mix until incorporated. Repeat this process, alternating between the dry mixture and the buttermilk, ending with the final third of the dry ingredients. Be careful not to overmix the batter. Mix only until the last streaks of flour disappear. Overmixing can develop the gluten in the flour, leading to tough, dense cakes instead of soft, pillowy ones.
6. Scoop the Batter: Using a small cookie scoop (about 1 tablespoon in size), drop rounded mounds of batter onto the prepared baking sheets. Be sure to space them about 2 inches apart, as they will spread while baking. A cookie scoop is highly recommended for creating uniform, consistently sized cakes.
7. Bake to Perfection: Place the baking sheets in the preheated oven and bake for 9-11 minutes. The cakes are done when the tops are puffed and spring back when you gently touch them. Be careful not to overbake, as this can dry them out.
8. Cool Completely: Remove the baking sheets from the oven and let the chocolate cakes cool on the sheets for about 5 minutes. This allows them to set slightly. Then, carefully transfer the cakes to a wire rack to cool completely. It is absolutely essential that the cakes are at room temperature before you add the filling; otherwise, the filling will melt and slide right out.

Part 2: Whipping Up the Fluffy Filling

1. Beat the Butter: While the cakes are cooling, prepare the filling. In a clean bowl of a stand mixer with the whisk attachment (or a large bowl with a hand mixer), beat the softened unsalted butter on medium-high speed for about 2 minutes until it’s very smooth and creamy.
2. Incorporate Sugar: Reduce the mixer speed to low and gradually add the confectioners’ sugar. Mix until it’s just combined, then increase the speed back to medium-high and beat for another 2-3 minutes until the mixture is light and fluffy.
3. Add the Magic Ingredients: Add the entire jar of marshmallow fluff, the vanilla extract, and the pinch of salt to the bowl. Beat on medium-high speed for another 3-4 minutes. The mixture will transform, becoming exceptionally light, glossy, and voluminous. This extended whipping time is key to the filling’s signature airy texture.

Part 3: Assembling Your Whoopie Pie Bites

1. Pair the Cakes: Once the chocolate cakes have cooled completely, turn half of them over so their flat bottoms are facing up. These will be the bases for your whoopie pie bites.
2. Fill the Bites: You can either use a spoon to drop a dollop of filling (about 1 to 1 ½ teaspoons) onto the center of each overturned cake, or for a neater look, transfer the filling to a piping bag fitted with a large round tip and pipe a swirl onto the cakes.
3. Create the Sandwich: Gently place a second cake, top-side up, on top of the filling and press down slightly until the filling spreads to the edges.
4. Serve or Chill: The whoopie pie bites can be served immediately for the softest texture. Alternatively, for a firmer filling, you can chill them in the refrigerator for about 30 minutes before serving.