Add the butter to a small saucepan. Place the pan over low heat. Cook until the butter is all melted. Stir with a whisk to help the process along.
Step 2
Next, add the arrowroot starch. Whisk well to incorporate. Keep whisking over low heat until the mixture starts to bubble.
Step 3
Immediately pour in the whole milk. Whisk constantly until the mixture thickens. Once it thickens (it will thicken all at once), season with ¼ tsp kosher salt.
Step 4
Feel free to add black pepper if you would like. Use immediately.