Combine the egg yolks, lemon juice, mustard and pepper in a blender cup. Cover and pulse for about 5 seconds;
Melt the butter in the microwave using a measuring cup (30 seconds, then 30 seconds more if needed). Butter needs to be completely melted and hot;
Set the blender to high speed, and with the blender running add the butter in a thin stream. Sauce will thicken immediately;
Keep sauce warm until serving. If you're not going to use it right away, pour the sauce in the measuring cup and leave it inside a pot filled with hot tap water.
Recipe by Easy Recipes, Etc™ at https://easyrecipesetc.com/hollandaise-sauce-recipe/