In a large bowl, combine 2½ teaspoons dry yeast with 120 ml warm water (about 43-46 °C / 110-115 °F). Let sit for 1 minute. Add 120 ml condensed milk, 75 g sugar, 75 g melted butter, 1 teaspoon salt, 1 egg, and roughly half of the 515 g flour. Mix until combined. Gradually add the remaining flour and ½ teaspoon cinnamon, mixing until a shaggy dough forms.
Knead the dough for 6-8 minutes until smooth and elastic. Place in a greased bowl and turn to coat with oil. Cover and let rise at room temperature for 1 hour until doubled in size.
While the dough rises, prepare the topping: In a bowl, beat together 135 g sugar and 110 g softened butter until creamy. Gradually mix in 30 g flour until a thick paste forms. Divide the paste in half. Mix 1 teaspoon vanilla extract into one half and 2 teaspoons cinnamon into the other half.
Divide the dough into 12 equal pieces and shape each into a ball. Place on a greased baking sheet, spacing them about 3 cm (1 inch) apart. Shape the vanilla topping into 6 small balls and the cinnamon topping into 6 small balls. Press each ball flat into a thin circle. Place one topping circle on each dough ball and score the topping with a knife: 2 cuts lengthwise and 2 cuts crosswise to create a grid pattern.
Cover the baking sheet loosely with plastic wrap and let rise at room temperature for 45 minutes until the rolls are puffy.
Preheat oven to 190 °C (375 °F). Bake for about 20 minutes until the conchas are golden brown on top.
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