To make hot chocolate: Bring the milk, water, and sugar just to the boil in a medium saucepan. Remove the pan from the heat and whisk in the chocolate. The hot chocolate needs to be blended at this point. At Ladurée, this is done with a hand-held blender (also called an immersion blender). If you have this tool, leave the hot chocolate in the saucepan and whip it with the hand-held blender for 1 minute. If you don't have a hand-held blender, transfer the chocolate to a traditional blender and whip on high speed for 1 minute. (When blending hot liquids, be sure to remove the center insert from the top of the blender, and cover the top with a folded kitchen towel to allow steam to escape.) The chocolate should be served immediately, while it is still very hot and frothy. Alternatively, you can pour the chocolate into a container to cool; the cooled chocolate can be reheated or served chilled. (The chocolate can be made up to 2 days ahead and kept tightly covered in the refrigerator.)
To reheat chilled hot chocolate: Working in a medium saucepan over low heat, warm the chocolate, stirring gently, just until the first bubble pops on the surface. Pull the pan from the heat, whip the chocolate for 1 minute with a hand-held blender (or in a traditional blender), and serve immediately.
To make cold hot chocolate: Chill the hot chocolate until it is very cold, then stir in 3/4 cup (200 grams) cold milk. Whip the cold chocolate for 1 minute with a hand-held blender (or in a traditional blender). Serve the cold hot chocolate over an ice cube or two.
Keeping: Once blended, the hot chocolate can be cooled and refrigerated in a tightly sealed jar for 2 days; reheat gently, or serve as cold hot chocolate.
Cold hot chocolate makes a fabulous ice cream float. Greenspan whips the cold chocolate to froth, pours it into a tall glass -- beer glasses are great for this -- and adds ice cream. While the traditional go-with-chocolate flavors, like vanilla, coffee, vanilla-fudge, and chocolate in all its renditions, are always good, her favorite floatable flavor is Ben & Jerry's Cherry Garcia.
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