First, make the Hollandaise sauce.
Fill a sauce pan half-full of water and choose a stainless steel mixing bowl that comfortably fits on top of the saucepan without touching the water. Bring water to a simmer.
Chop up butter and set aside. I’ve had best results putting it back in the refrigerator, but it might make for less work to let it soften.
Whisk the yolks and water in the bowl until yolks lighten a lot.
Add the sugar, which to combine. Then whisk over heat continuously until yolks have thickened to the point that your whisk can easily create figure-eights with the ribbons dripping off of it. Remove from heat and whisk the butter in, one or two pieces at a time. Return the yolks to the heat as needed to keep it hot enough to melt the butter. Yes, your arm will want to fall off, but it will be worth it!
When the butter is fully incorporated, add cayenne.
Whisk to incorporate, then cover with a towel. When you are ready to serve, return to heat, whisk like crazy and add a couple of drops of water to revive the sauce. Do not count on it surviving more than 20 or 30 minutes.
Make 1 piece of toast per person. Cut into 2 or 4 triangles and place on the plate.
Fill a large, high sided skillet with water and bring to a boil. Add one cup of lemon juice or vinegar per cup of water. When the water is boiling carefully crack the eggs into a small bowl, one at a time, and gently tip the bowl into the skillet. When all the eggs are in, remove from heat and cover. If you like the eggs really runny, give them 3 minutes or so. A perfect soft boiled interior arrives in 6 minutes or so.
While the eggs are cooking, toss several pieces of prosciutto into a non-stick pan over medium high heat until they just start releasing fat. Remove from heat.
With a slotted spoon, remove the eggs carefully, shake off excess water, and carefully place on the toast points. Lay the prosciutto over the eggs. Revive the Hollandaise and put several big spoonfulls over the top.
Spoon a dollop of caviar on the top and serve.