Start by pouring the whole milk into a saucepan. Heat it gently over medium heat until steam begins to rise and small bubbles form around the edges. Scalding the milk helps to develop a deeper flavor in your sauce. Keep an eye on it, so it doesn't boil—just like a good friend, it needs gentle management!

In a mixing bowl, combine the sugar, brown sugar, and cornstarch. Then, add the egg yolks. Whisk them together until the mixture is pale and slightly thickened. This process not only mixes the ingredients but also introduces air, which is important for the texture.

Now, let’s combine our efforts without making an omelet. Gradually pour the hot milk into the egg mixture while constantly whisking. This step is key; rushing in the hot liquid can solidify the eggs too quickly, resulting in scrambled eggs. We want a smooth crème anglaise, not breakfast!

Pour the combined mixture back into the saucepan. Place it on low heat and stir continuously with a wooden spoon or heat-resistant spatula. Cook for 5-10 minutes until the mixture thickens and coats the back of the spoon. It should reach about 170°F to 175°F (go ahead, use that kitchen thermometer!).

Once thickened, remove the custard from heat. Strain it through a fine mesh sieve into a bowl to remove any cooked bits or the vanilla pod, if you used one. Stir in the vanilla extract, Cognac, and cinnamon. Your crème anglaise is complete!