Preheat oven to 375°F. Grease molds and prepare with parchment paper.
In a mixer, add flour, sugar, eggs, salt, and the sponge.
Beat until the dough starts to pull away from the sides, at least 13 minutes.
Add butter gradually until it incorporates completely and the dough pulls away from the sides.
Cover the buttered dough in a bowl and let it ferment for 60 minutes or until it doubles in size.
Deflate the dough and divide it into two equal parts.
Divide each part into 6. Flatten each part into a rectangle, fold it like a letter, flatten it again, and roll each one of the parts.
Place the 6 rolls into a mold.
Cover and let it double in size.
Brush with egg yolk and bake on the middle rack for at least 30 minutes or until completely golden brown.
Remove from the oven and let cool.
