Bloom the Yeast
In a small bowl, combine warmed milk and active dry yeast
Let it rest for 5 minutes, until bubbly or foamy
Mix the Dough
In a large mixing bowl, whisk together bread flour, all-purpose flour, granulated sugar, and salt
Make a well in the center and add the bloomed yeast, eggs, butter, and optional vanilla extract
Stir until a sticky dough forms. If you prefer, use a stand mixer fitted with a dough hook. Otherwise, knead it by hand on a lightly floured surface
First Rise
Form the dough into a ball. Lightly oil a large bowl and place the dough inside.
Cover the bowl with plastic wrap or a clean kitchen towel.
Let the dough rise in a warm spot until doubled in size (about 60 minutes).
Prepare the Topping
In a medium bowl, mix all-purpose flour, granulated sugar, and softened butter.
If making two flavors, separate the mixture into halves: add cocoa powder to one half, and vanilla extract to the other.
Work each portion into a smooth paste that you can press to flatten over the shaped dough.
Shape the Conchas
Punch down the dough and divide the dough into 12 equal portions.
Roll each piece into a round dough ball.
Press a flattened portion of the topping onto each dough ball. Use a paring knife or concha cutter to score a shell pattern.
Second Rise and Baking
Transfer dough balls to a baking sheet lined with parchment paper. Cover lightly and let them rest for about 30 minutes.
Preheat your oven to 350°F (175°C).
Bake for 15–18 minutes or until the conchas are lightly golden. Remove and let them cool slightly on a rack.