Note: Microwave oven intensity varies. You may need to adjust the cooking times accordingly.
In a small glass bowl, microwave butter on high until just melted (about 30 seconds). Stir in salt, and cayenne.
In a second small glass bowl, whisk water, lemon juice, and egg yolk until thoroughly blended. Immediately whisk this into the butter mixture (not the other way around) until completely combined.
Micro-cook this mixture for five seconds on high, then stir thoroughly. Repeat this two- to four times until the mixture just barely begins to thicken, then remove from microwave (the sauce will continue to thicken).
It is important NOT to overcook this Hollandaise, as it will continue cooking and thickening after removed from the microwave. Stop the micro-cooking at the first sign that the sauce is thickening.
Four turns of five-seconds-then-stir is all it takes in my own microwave oven (yours may vary).
Makes 1/4 cup, and doubles or quadruples easily (with different cooking time). Keeps in refrigerator for 48 hours.