1) For the hollandaise: in a small saucepan over low heat, melt the butter until bubbling hot. In a blender, add the egg yolks, black pepper, to taste, and one tablespoon of lemon juice. Cover the blender and blend on high speed for several seconds. Either remove center cap of the blender lid or carefully remove the lid itself with blender still running. Pour the hot butter in a thin, steady stream into whirling egg mixture. Add the chipotle and pulse to combine. Taste sauce and adjust seasoning with lemon juice, salt, and pepper (if your sauce gets too thick, you can thin it out with a couple tablespoons of boiling hot water).
2) For the eggs: in a heavy saucepan combine about 2 3/4 L of water and the vinegar and bring to a simmer. Break one egg into a small bowl or cup and slide the egg into a ladle. Lower the ladle into the water, and hold it there, for one minute before removing the ladle and leaving the egg to cook for two minutes more. Poach at a low simmer until the whites are firm and the yolks are still runny. Repeat with each remaining egg. Transfer the eggs, with a slotted spoon, to paper towels until ready to serve.
3) To assemble: arrange a croissant half, cut side up, on each of four plates. Put one piece of prosciutto on each half, then two poached eggs and top with hollandaise. Serve immediately.
RAW EGG WARNING
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.
Cook's Note: the chipotle chilli is a smoked, dried jalapeno. They can be found dried, pickled or canned "in adobo" which is a sauce made with ground chiles, herbs, vinegar and spices. The sauce tends to be more mild than the chipotles themselves and can be used in place of them to reduce the heat in a dish. Both dried and canned chipotles are easily found in most supermarkets.