In Batter Bowl #1, whisk together the eggs yolks and 6 tbsp of the granulated sugar until thick and slightly foamy, about 30 seconds. Set aside.
In the saucepan, whisk together the cocoa powder, the special dark cocoa powder, the cinnamon, and the remaining 6 tbsp of sugar. Measure out ¼ cup of the milk and whisk it into the saucepan with the cocoa powder, etc. Be sure to scrape the bottom edges of the saucepan to get all the dry ingredients mixed into the wet. When all is combined, the mixture should be smooth and dark, like brownie batter.
Whisk in the remaining ¾ cup of milk, the cream, vanilla extract, and salt; this mixture will be lighter in color, like chocolate milk, and slightly thick.
Place the saucepan over medium-high heat and warm the mixture up until it’s just starting to simmer; this means tiny bubbles should just start appearing around the edges of the saucepan. The mixture will de-thicken right before it begins to simmer. This will take about 10 minutes.
Turn off the heat and remove the saucepan from stove to Potholder #2, next to Batter Bowl #1 containing the egg yolks.
Use the soup ladle to carefully scoop out some hot liquid from the saucepan. Whisk the egg yolks to break them up a bit, then slowly stream the hot liquid into the egg yolks, whisking briskly the entire time to prevent the egg yolks from curdling. Once the first scoop is thoroughly whisked in, repeat process two more times, whisking the entire time to prevent the egg yolks from curdling.
Pour the egg yolk mixture from Batter Bowl #1 into the saucepan, once again whisking the entire time. All ingredients should now be in the saucepan.
Return the saucepan to the stove on medium heat. While continuously stirring with a rubber spatula, heat the mixture until it thickens and thickly coats the back of the spatula, about 5 minutes. When it’s ready, you should be able to create a clear trail when you run your finger on the back of the spatula.
Carefully pour the hot liquid into the fine mesh strainer placed over Batter Bowl #2, which is placed on Potholder #3. Push liquid around with the spatula to make sure it all strains into Batter Bowl #2.
Place Batter Bowl #2 gently into Ice Water Bowl, making sure that the bottom of Batter Bowl #2 touches the ice water but does not allow any ice water to get into the Batter Bowl.
Stir ice cream base in Batter Bowl #2 occasionally until the base has cooled.
Once ice cream base is cooled, cover Batter Bowl #2 with plastic wrap and refrigerate for 12-24 hours.