For the chocolate cake: Preheat the oven to 350 degrees F (180 degrees C). Line a baking sheet with parchment paper.
Combine the flour, cocoa, baking powder and salt in a bowl.
Beat the sugar, butter and eggs with a mixer for about 30 seconds. Add the vanilla and beat for 30 seconds more. Add the flour mixture to the sugar mixture, alternating with the milk and beating until smooth.
Using an ice cream scoop, drop mounds of the batter (about 1/2 cup for each) onto the lined baking sheet (6 per sheet as they will spread when baking). Bake for 15 minutes. Let cool completely on a wire rack. Repeat with the remaining batter.
For the marshmallow filling: Whisk together the sugar, honey, vanilla, egg whites and 2 tablespoons water (30ml) in a large heatproof bowl using a hand mixer. Place the bowl over a saucepan of simmering water. Beat with the mixer until the mixture is thick, creamy and stands in peaks, 5 to 7 minutes. Remove from the heat and beat for a minute or so more, then let cool completely. (You can keep the marshmallow filling in the refrigerator until ready to use.)
Spread a generous amount of filling on the flat side (bottom) of a chocolate cake. Sandwich with another cake, pressing down gently to spread the filling evenly. Repeat to make 8 whoopie pies. Serve immediately or wrap the whoopie pies individually in plastic wrap (they can be frozen).