STEP 1:
Combine flour, instant yeast, & sugar until dispersed. With hook attachment, & mixer to medium-low speed, add in egg, egg yolk, milk & vanilla. Then mix in 1-2 Tbsp of butter at a time.
STEP 2:
Let it knead for 5-10 minutes, until the dough feels smooth & elastic. Place it to rest on a floured surface for 10 minutes. Then roll the dough into a 12x16 inch rectangle.
STEP 3:
Spread butter onto the dough. Mix sugar, cocoa powder & cinnamon & sprinkle over the butter. Sprinkle chocolate chunks on top, roll it into a log, and slice into 12 large rolls.
STEP 4:
Place the rolls in a prepared muffin tin & cover with plastic wrap. Place them in a warm environment for 45 minutes to rise until they've doubled in size.