Infuse the milk; Pour 3 cups of milk into a heavy-bottomed saucepan. Add the cinnamon sticks and heat over medium until the milk just begins to steam. Don't let it boil. Remove from heat and let steep for 10 minutes.
3 cups whole milk, 2 cinnamon sticks
Make the egg mixture. In a bowl, whisk together the egg yolks, sugar, cornstarch, and salt until smooth. Slowly whisk in the remaining 1 cup of cold milk until no lumps remain.
4 egg yolks, 3/4 cup sugar, 3 tablespoons cornstarch, Pinch of salt, 1 cup whole milk
Temper the eggs. Remove the cinnamon sticks from the warm milk. Ladle about 1/2 cup of the warm milk into the egg mixture while whisking constantly. This prevents the eggs from scrambling.
Cook the custard. Pour the tempered egg mixture back into the saucepan with the remaining warm milk. Cook over medium-low heat, stirring constantly with a wooden spoon or silicone spatula. Don't stop stirring.
Watch for thickness. After 8-10 minutes, the custard will thicken enough to coat the back of a spoon. Draw your finger across the spoon. If the line holds without dripping, it's ready.
Finish and cool. Remove from heat immediately. Stir in the vanilla. Pour into small cups or a serving bowl. Press plastic wrap directly onto the surface to prevent a skin from forming.
1 teaspoon vanilla extract
Chill. Refrigerate at least 2 hours. Dust with ground cinnamon before serving.