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 1 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.) For the eggs: In a heavy saucepan combine about 2 quarts of water and the vinegar and bring to a simmer. Break 1 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 1 minute before removing the ladle and leaving the egg to cook for 2 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. To assemble: Arrange a croissant half, cut side up, on each of 4 plates. Put 1 piece of prosciutto on each half, then 2 poached eggs and top with hollandaise. Serve immediately.
**The chipotle chile 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.;
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.