Preheat an air fryer to 200 degrees F (93 degrees C) according to manufacturer's instructions.
Use a fork to blend the butter and sugar together in a medium bowl. Mix in flour and combine until smooth, reaching a cookie dough-like texture.
Spread the butter mixture evenly over the slice of milk bread, smearing until smooth. Use a knife to score the bread topping with criss-cross lines that resemble a pineapple skin. In a small bowl, beat the egg to form an egg wash. Carefully spread a thin layer over the bread topping.
Cook the bread in the preheated air fryer for 4 minutes; turn up the air fryer temperature to 400 degrees F (200 degrees C). Continue to cook bread slice until toasted, 3 to 4 minutes. Serve immediately.