Add 2 cups heavy cream, 1 cup whole milk, 1½ tsp ground cinnamon, and ½ cup packed brown sugar to a medium sauce pan. Whisk to combine. Drop in the cinnamon stick.
Bring cream mixture to a simmer, stirring continuously. When you see bubbles forming at the edges, give it another 1-2 minutes (it should never come to a boil). Remove from the heat.
Add the 5 egg yolks to a medium bowl and whisk together.
Add ½ cup of the cream mixture into the eggs, making sure to whisk the entire time. Give it a minute, then repeat this procedure 2x more. Here, we're trying to bring the eggs up to temperature without scrambling them.
Once you have a smooth consistency, slowly pour the mixture back into the remaining cream mixture, whisking the entire time.
Cook over medium heat until the mixture is thick enough to coat the back of a spoon. That means when you dip a spoon into the mixture, the entire back will remain coated evenly, even after excess drips off.
Optional: It can help the texture of the custard to strain it through a fine mesh strainer. Especially if you see a few small lumps that might be eggy (as opposed to clumps of cinnamon). If you're a custard pro and don't have lumps, you may skip this step.
Add to a storage container. My batch was about 3½ cups or 850ml of liquid (3 cups and 800ml after straining) and fit in a 24oz mason jar. Refrigerate at least 4 hours or overnight.
Add the chilled base to the canister of your ice cream churn and churn according to manufacturer's instructions.
You can eat it immediately, but the mixture will have more of a soft-serve texture. For a harder ice cream, add to a freezer-safe container and freeze for 2+ hours or until desired consistency.