Preheat oven to 350 degrees.
Pour water to a depth of 1 inch into bottom of a double boiler over medium heat; bring to a boil. Reduce heat, and simmer; place chocolate and butter in top of double boiler over simmering water. Cook, stirring occasionally, 5 to 6 minutes or until melted. Remove mixture from heat, and cool 10 minutes.
Whisk together sugar, flour, and eggs in a large bowl; add chocolate mixture, stirring just until combined. Stir in vanilla and salt.
Place paper baking cups in 2 (12-cup) muffin pans, and coat with cooking spray; spoon batter into cups, filling two-thirds full.
Bake for 12 to 15 minutes. Be careful not to over bake; they should be gooey. (They will crack and sink in the middle; this is normal.) Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely.
Sift powdered sugar over the top of each cupcake.