In a mixing bowl, combine warm water, warm milk, yeast, and a teaspoon of sugar. Let it sit for 5-10 minutes until frothy.
Add the flour, sugar, salt, eggs, and butter to the yeast mixture. Knead until smooth and elastic, about 10 minutes.
Place the dough in a greased bowl, cover, and let it rise until doubled in size, about 1-2 hours.
For the topping, mix sugar, butter, flour, and vanilla extract until smooth. Divide into 12 portions.
Divide the dough into 12 balls. Place on a baking sheet. Flatten each ball slightly and place a portion of topping on each. Score the topping with a knife to create a shell pattern.
Preheat the oven to 350°F (175°C). Bake for 15-20 minutes or until golden brown.