Blender Hollandaise

0 Place butter in a small saucepan over medium-low heat. Heat, stirring frequently, until the butter is just melted. Reduce the heat if the butter starts to bubble or steam. Maintain the melted butter at this low but warm temperature while you prepare the blender.
1 In the bowl of a blender (see notes below for tips), combine egg yolks, lemon juice, salt, and cayenne pepper (if using). 
2 Put the lid on the blender and pulse the mixture for a couple seconds until everything is combined. 
3 Using whatever method works best for your blender (this may involve moving the lid so it is slightly ajar or removing part of the lid to create an opening, turn on the blender and slowly pour the butter into the eggs while the blender is running. (Stop just short of adding all of the butter, so that you can discard any solids that separated out of the butter while it was melting.) 
4 After the butter has been added, stop the blender and taste your sauce. Add some more lemon juice or salt, if needed. 
5 If the sauce is thicker than you’d like, add some warm water, 1 teaspoon at a time, and blend until it reaches the thickness you would like.
6 Serve immediately.