Mix flour, yeast, sugar, and salt in a bowl.
Stir warm milk, melted butter, and egg into dry ingredients until a soft dough forms.
Knead briefly on a floured surface for about 5 minutes.
Let dough rest 20 minutes while you mix topping: blend powdered sugar, softened butter, 1/3 cup flour, and vanilla into a paste.
Divide dough into 8 balls, place on a baking sheet, and flatten tops slightly.
Spread a thin layer of topping on each bun and use a knife to score a shell pattern.
Bake at 375°F (190°C) for 12–15 minutes until golden.