Stir cornstarch in cup of warm milk until dissolved.
In the bowl of a stand mixer, combine milk mixture, yeast, and 2 Tbsp of the sugar. Gently stir together and let sit for 8-10 minutes until frothy.
Add melted butter (cooled to room temp.), the rest of the sugar, salt, vanilla and orange zest to yeast mixture.
Using the dough hook attachment mix on medium low speed.
Slowly spoon in flour on medium speed, and then turn up to medium high until dough has formed.
The dough should be very slightly tacky, with a soft and smooth texture.
Once formed, let the dough rest for about 10 minutes.
Turn mixer back on to medium high and beat for 6-8 minutes.
(Tear off a small piece of dough and stretch out with your fingers. A simple "window pane" test will let you know if the dough is ready. If not, beat for another 2-3 minutes and test again.)
Remove dough from bowl and place on a lightly floured surface.
Weigh your dough and divide into two halves.
Lightly dust one half with flour, wrap in plastic wrap and place in the fridge.
Take the second half and evenly divide it into two halves.
Make Chocolate Pastes
Use a separate bowl for each paste.
Stir cocoa powder and milk together with a spoon.
You should have one thick chocolate paste and one light chocolate paste.
Place one of the remaining halves of dough in each bowl of paste.
Work the chocolate into the dough with your hands. Kneading to incorporate all the chocolate into each dough.
You should now have one dark chocolate ball of dough and one light chocolate ball of dough.
Lightly flour each ball, cover in plastic wrap and set in the fridge.
Let all three dough balls chill for about 60-90 minutes.