Whisk flour, salt, sugar, milk, eggs, the 2 tablespoons melted butter and orange zest until smooth. Refrigerate, covered, for at least an hour and up to overnight.
Heat a 6- or 8-inch nonstick skillet over medium heat for a minute or two. Add a small pat of butter. Stir the batter with a ladle; add a couple of tablespoons to the skillet. Swirl it around so it forms a thin layer on bottom of pan; pour excess back into the bowl.
When the top of the crepe is dry, about a minute, turn and cook the other side for 15 to 30 seconds. (The crepe should brown only very slightly and not become at all crisp.) Don'tworry if the first crepe doesn't work; simply discard it if necessary-there is plenty of batter.
As the crepes are done, stack them in a single layer with a sheet of waxed paper in between each. Repeat the process, adding butter to the skillet and adjusting the heat as needed, until all the batter is used. Crepes can be made up to a day ahead; cover with plastic wrap and refrigerate.
Place chocolate in small bowl. Heat cream in small saucepan over medium heat until just simmering. Pour over chocolate. Let stand 2 minutes to melt. Stir until smooth. Serve immediately or let cool, then reheat briefly in microwave oven.
To serve, heat oven to 200°F. Reheat stacked crepes at 200°F for 15 minutes. Fold crepes in half, then in half again. Fan onto heated serving platter, then serve with warm sauce on the side.