Preheat your air fryer to 175°C.
Slit the slider buns slightly on top or on the side and stuff each bun with a cube of salted butter.
In a bowl, cream together the softened butter and sugar until smooth.
Add the egg and vanilla extract and mix well.
Dissolve the coffee powder in the boiling water, then add it to the mixture.
Fold in the flour and mix until you get a smooth coffee batter.
Transfer the mixture into a piping bag (or ziplock bag), then pipe a swirl generously over each bun.
Air fry for 8–10 minutes at 175°C, or until the topping is cooked and lightly crisp on top.Serve warm while the butter inside is still melted and gooey.