1. Preheat oven to 375°F Liberally spray six (6-ounce) ramekins with nonstick spray.
2. In a small saucepan over low heat melt the chocolate and butter, stirring often to prevent burning; remove from heat and cool.
3. In a large mixing bowl, beat eggs and sugar with electric hand mixer until ribbon stage, approximately 7 minutes; At that stage, it will be pale yellow and look like lightly whipped cream.
4. Add flour, cocoa powder, vanilla extract, and salt.
5. Beat the batter for 2 additional minutes; add the melted chocolate mixture and beat another 5 minutes (The batter needs a lot of beating; this incorporates air, lightening the cake's texture).
6. Scoop the batter into the prepared ramekins, approximately 3/4 cup of batter in each ramekin.
7. NOTE: At this point, the cakes can be refrigerated and then baked later.
8. When ready to bake, place the ramekins on a baking sheet and bake 15 to 17 minutes or until puffed and mostly dry on top around edges.
9. NOTE: It's critical to bake these cakes for the right amount of time otherwise, they'll either be way too molten, or end up as brownies, oven temperatures vary, so check the cakes after 15 minutes; if they're really moist on top, bake one minute and check again.
10. Remove cakes from oven and let rest 3 minutes (it's okay if they sink a little).
11. To unmold, place serving plate on top of ramekin and invert.
12. Carefully remove ramekin, tapping lightly on the plate to loosen cake.