In a mixing bowl, mix together the flour, cocoa powder, sugar, and salt, and set aside.
In another bowl, crack and beat the eggs.
Mix the milk and vanilla into the eggs until combined.
Add the flour mixture to the liquid and mix until it's combined and looks something like chocolate milk.
Let this mixture sit, covered, for 20 minutes to allow the flour to fully absorb the liquid.
Preheat the oven to 400℉ with the skillet inside, so the skillet is also preheated. Use caution handling this!
Stir the batter to release any air bubbles.
Add the butter to the skillet to coat the bottom, then pour the batter in the center.
Bake for 18 minutes.
Remove the skillet from the oven and let cool for at least 10 minutes. The dutch baby will be very inflated, but don't worry; it will collapse back down into a delicious cake.
Top with your choice of toppings and enjoy!