Preheat Oven: Heat oven to 390°F (200°C).
Boil Butter Mixture: In a pot, mix butter, milk, water, salt, and sugar. Bring to a boil, ensuring the butter melts completely.
¼ cup water, ¼ cup milk, 3 tbsp unsalted butter, ¼ tsp salt, 1¼ tsp sugar
Add Flour: Off the heat, add flour and cocoa powder (mixed) all at once and stir. Return to low heat, stirring continuously for 2-3 minutes until the dough is thick, glossy, and leaves a thin film on the pot bottom.
¼ oz cocoa powder, 2¼ oz all-purpose flour
Cool Dough: Transfer dough to a bowl and let it cool for 10 minutes, spreading it out to cool faster.
Whisk Eggs: Beat eggs in a separate bowl.
2 medium eggs
Mix Dough: With the dough slightly cooled, mix with a hand mixer. Gradually add half the eggs, mix, then add the remaining eggs bit by bit until the dough forms a 'V' shape when lifted with a whisk or spatula.