Melt the butter. Melt 8 tablespoons of butter in the microwave. Remove the butter from the microwave and let it stand for about 30 to 40 seconds.
Combine. In the meantime, add the egg yolks, lemon juice, salt, cayenne pepper, and mustard to a wide-mouthed mason jar.
Blend. Grab the immersion blender and stick it inside the jar. Pour the butter into the egg yolk mixture in a thin stream, blending as you go. While blending, move the immersion blender up and down, slowly, so as to combine everything together and to thicken. This should only take about 30 seconds.
Taste and adjust. Remove the blender and taste the sauce for seasonings and flavor; adjust accordingly. Also, if the texture is too thick, you can add a couple of teaspoons of water or lemon juice and stir with a fork until combined. The sauce will thicken as it stands; stir in more liquid as needed.
Store. Use your Hollandaise sauce within 1 to 2 hours of making; refrigerate any leftover sauce after 2 hours.
Notes
STORING HOLLANDAISE SAUCE
Cover freshly made Hollandaise Sauce with plastic wrap to keep it warm. Use within 1 to 2 hours.
Refrigerate any leftover sauce after 2 hours of cooking.