Place the butter in a small saucepan set over medium-low heat. Warm just until sizzling. Add the egg yolks, lemon juice, salt, and cayenne (if using) to a blender. Blend at medium speed for about 1 minute, until well-blended and yolks are pale. With the blender running on medium, very slowly and carefully stream the butter into the egg yolk mixture. The yolks will cook thanks to the heat of the butter, and the sauce will form. It will be pale yellow and fairly runny. Pour into a bowl and keep in a warm place until ready to serve. I usually set mine over a bowl of warm water.
To Make the Eggs Benedict:
Toast the English muffins and butter them. Lay two toasted English muffins each, buttered side up, onto four serving plates. Layer avocado, then roasted red peppers, over the top.
Now, poach the eggs. Fill a shallow (about 3-inch) saucepan about ¾ of the way full with water and set it over high heat. Stir in a pinch of salt and 1 tablespoon of vinegar and bring to a boil. Meanwhile, crack four of the eggs into individual bowls or teacups. Gently lower the teacups into the water, allowing the water to creep a bit into the cup before pouring the egg completely into the water. Repeat as quickly as possible with remaining three eggs. Cook for 3 minutes exactly. (Set a timer!) Be sure not to let the water boil over and if any white foam appears, gently skim it off with a spoon. Once the 3 minutes is up, scoop the poached eggs out of the water with a slotted spoon and set one egg on top of each muffin. To poach the remaining four eggs, discard water and start with fresh water, salt, and the remaining tablespoon of vinegar.
Once the Eggs Benedict are assembled, drizzle Hollandaise sauce over each and, if desired, sprinkle with additional cayenne or garnish with a basil leaf or chopped chives. Serve immediately.