Pour milk into the bowl of a stand mixer¹ that’s been fitted with a dough hook. Add yeast and 1 teaspoon sugar and stir. Allow to sit for 5 minutes or until yeast is foamy.
Add remaining ¼ cup (50g) granulated sugar, melted butter, egg and egg yolk and stir until combined.
In a separate bowl, whisk together 2 ¾ cups (345g) all purpose flour, cornstarch, and salt. With mixer on low speed, gradually add flour mixture (my dough hook has trouble reaching the sides of the bowl, so usually I’ll have to stir by hand with a spoon to get the flour incorporated until I’ve added about half of the flour, then the dough hook functions well).
Continue to stir on low speed until the dough clings to itself to form a ball that pulls away from the side of the bowl. It should be smooth, elastic, and slightly tacky (but not sticky) to the touch. Add additional flour as needed until this dough consistency is reached.
Once proper dough consistency is reached, transfer to a lightly oiled bowl. Cover tightly with plastic wrap and place in a warm, draft-free place to rise until doubled in size (about 2 hours).
When dough has risen, prepare your pie plate and filling.
FILLING & ASSEMBLY
Melt 1 ½ Tablespoons of the butter and place in a 10” pie plate. Set aside.
Combine remaining 4 tablespoons butter, granulated sugar, cocoa powder, and cinnamon, if using. Stir together until completely combined. Set aside, you’ll need this momentarily.
Take risen dough and gently punch down/deflate the dough and transfer to a clean surface. Use a rolling pin to roll into a 15x9” rectangle.
Spread prepared chocolate filling over the surface of the dough, leaving about ½” perimeter around the edges. Sprinkle finely chopped chocolate or chocolate chips over the filling.
Working with one 15” side, tightly roll the dough into a log. Cut into 9 even slices. Transfer to prepared pie plate and cover with a clean towel (or plastic wrap or foil). Place in a warm, dry place and allow to rise until puffy and almost doubled in size (about 30-60 minutes). Meanwhile, preheat oven to 350F (175C).
Once rolls have finished their second rise and oven is preheated, remove towel (or foil or plastic wrap) and transfer to oven. Bake 20-24 minutes or until rolls are cooked through and lightly golden brown.
Allow to cool for at least 20 minutes before covering with chocolate icing.
Whisk together powdered sugar, cocoa powder, 2 tablespoons whole milk, and vanilla extract. Add additional milk, as needed, until desired consistency is reached.
Drizzle icing over rolls. Enjoy!
¹It is possible to make this dough without a stand mixer, you can do it by hand in a large bowl but it will take a bit of an arm workout. Rather than mixing with a dough hook on low speed I would recommend kneading on a clean surface until the indicated consistency is reached (smooth, elastic, slightly tacky to the touch but not sticky).
Make in advance or freeze: See notes in post (blue box) for how to make dough in advance or freeze.