Cilantro Hollandaise

0 Melt butter either in a saucepan over medium heat or place in a measuring cup and microwave for 1 minute, until fully liquid.
1 In a cup that just barely fits the head of an immersion blender, add the egg yolk, water, and lemon juice.
2 Place the immersion blender in the cup and turn on. Slowly stream in the hot butter with the blender constantly running until all butter is added. The sauce should be emulsified and thick.
3 Add the cilantro to the sauce and blend.
4 Serve warm.