Steps
1. In a small saucepan, combine butter, water, and salt. Heat until butter melts and the mixture starts to boil.
2. Remove from heat and quickly stir in the flour until a smooth dough forms.
3. Return to low heat and stir for 1-2 minutes to dry the dough slightly.
4. Transfer dough to a bowl and let cool for 5 minutes.
5. Beat in the egg one at a time until the dough is smooth and shiny.
6. Preheat airfryer to 180°C (350°F).
7. Pipe or spoon small rounds of dough onto parchment paper sized to fit your airfryer basket.
8. Airfry for 12-15 minutes until puffed and golden brown. Avoid opening the basket during cooking.
9. Meanwhile, to make custard: whisk egg yolk, sugar, and cornstarch in a bowl.
10. Heat milk in a saucepan until warm but not boiling.
11. Slowly pour warm milk into egg mixture while whisking.
12. Return mixture to saucepan and cook on low heat, stirring constantly until thickened.
13. Remove from heat, stir in vanilla extract, and let cool.
14. Once puffs are cool, slice them halfway and fill with custard using a spoon or piping bag.
15. Serve immediately or chill before serving.