Meanwhile, add the cream, Dijon mustard, garlic powder and cayenne to a small saucepan. Whisk to combine.
Heat the cream mixture over medium heat. When the cream starts to bubble around the edges, lower the heat to low.
Stir in the shredded cheddar, 1/3 at a time, slowly and patiently stirring it in with a rubber spatula until it is completely melted. Don't stir vigorously or the sauce will become stringy. Turn the heat off as soon as it's all melted.
Pour the cheese sauce over the steamed broccoli and serve.
Using store-bought pre-shredded cheese seems to help with the sauce's texture. I suspect that the small amount of anti-caking agent typically added to shredded cheese (powdered cellulose in the Whole Foods brand) helps the sauce thicken.
If you're shredding your own cheese and you feel that the cheese sauce is too thin, simply stir in a bit more cheese - try 1/4 cup more. You can also mix 1 teaspoon cornstarch with 1 teaspoon of cold water and stir that into the sauce. This will add a tiny amount of carbs - 2 grams for the entire recipe.
Heat the sauce over low heat and stir the cheese in slowly and gently to prevent the cheese from becoming stringy.
Please don't skip the mustard - its acidty helps improve the sauce's textue.