Dissolve the yeast in warm water in the bowl of a standing mixer fitted with the paddle attachment.
Add the milk, sugar, butter, salt, egg and 2 cups of flour. Stir together until just combined. Switch to the dough hook attachment.
Add an additional cup of flour and knead on medium-low speed. When the flour is incorporated, knead at medium speed for 5 minutes. The dough should be elastic and slightly sticky but easy to handle. If the dough is very sticky, knead in the additional flour. Place the dough in a large greased bowl and turn the dough over to coat. Cover with plastic wrap and allow it to rise in a warm place until doubled, about 1 1/2 hours.
In the meantime, make the sugar paste topping: Beat the sugar, margarine and vanilla together until light and fluffy. Stir in flour and mix until a thick dough forms. Add additional flour if needed. Divide dough into 3 or 4 even pieces and tint each with food color. If the dough becomes sticky from the food color, add more flour. Cover pieces with plastic wrap until ready for use.
When the dough ready, turn it out onto a lightly floured surface. Divide the dough into 16 even pieces (I did this with my bench scraper). Shape each dough piece into a ball by tucking the corners under (don’t roll between your palms, this will just deflate the dough and make it tough). Place the dough buns ona large Silpat or parchment-lined baking sheet.
Roll out the sugar paste pieces on a lightly floured surface. Cut into 2 1/2-inch rounds with a cookie cutter or pastry ring. Use the pastry ring to score lines into the paste to resemble the ridges on a seashell (concha). Transfer the scored sugar paste rounds to the buns using an offset spatula. If the paste doesn’t want to adhere naturally, use a pastry brush to apply dots of water on the underside before applying to the buns. Allow the buns to rise on the baking sheet for 40 minutes.
Preheat oven to 375°F. Bake the buns for 18-20 minutes, or until they are fragrant and lightly brown on the bottoms.