Step 1In a blender, pour in three-quarters cup milk, one-half cup water and the eggs, and pulse two or three times until smooth. Add the salt and flour. Blend the batter until smooth. With the blender running, add the melted butter. The batter should be as thick as heavy cream. If necessary, add up to one-quarter cup more milk to get the desired thickness. Strain the batter into a bowl, stir in the chives, cover and refrigerate at least 1 hour.
Step 2Heat an 8- to 9-inch nonstick skillet or crepe pan over medium-high heat until a few drops of water skitter when sprinkled on the surface. Lightly grease the pan with a thin film of butter and return it to the heat.
Step 3Stir the batter to redistribute the chives and pour one-fourth cup into the center of the skillet. Swirl the skillet quickly so that the batter completely covers the bottom. Pour the excess batter back into the bowl, trimming with a spatula any ragged edges before they set. Return the skillet to the heat and cook until the surface of the crepe firms and dries, about 1 minute. Using a small spatula or table knife, gently lift the edges of the crepe. Grab the edges with your fingertips and quickly flip over the crepe; the cooked side should be lightly colored. Cook the uncooked side just until the crepe slides free in the pan, about 30 seconds. Remove the crepe to a plate.
Step 4Repeat until all of the batter is used, adding more butter as needed. Layer the crepes with a piece of wax paper between each to prevent sticking. You should have at least 12 crepes. The crepes can be stored, tightly sealed with plastic wrap, in the refrigerator, as long as you have wax paper between them.
Step 1Reheat the crepes: Heat the oven to 325 degrees and wrap the crepes in aluminum foil, leaving the wax paper in between. Place on a cookie sheet in the oven until heated through, about 15 minutes. Remove the crepes from the oven and keep warm in the foil. Don't overcook them or they'll dry out.
Step 2While the crepes are warming, melt the butter in a large skillet over medium heat. Add the prosciutto and cook until softened, about 3 minutes. Add the shallots and cook until softened, about 2 minutes.
Step 3Increase the heat to medium-high and add the mushrooms. Season with one-eighth teaspoon salt and cook, stirring often, until the mushrooms release their liquid, 3 to 4 minutes. Keep cooking until the mushrooms reabsorb the liquid and tighten up, about 5 more minutes. Add a dash of freshly ground black pepper.
Step 4Add the white wine and cook until it is reduced to a syrup, about 3 minutes. Add the whipping cream and reduce slightly until it achieves a sauce-like consistency, about 2 minutes. Stir in the chopped hazelnuts and 1 1/2 tablespoons chives and season to taste.
Step 5Spoon about 3 tablespoons of filling in the center of a crepe. Fold the crepe in half and then into quarters, gently pressing the filling so that it spreads toward the edges. Repeat with the remaining crepes. Arrange them two to a plate, garnish with creme fraiche and the remaining chives and serve immediately.