Preheat oven to 325 degrees F (165 degrees C). Line 24 muffin cups with paper liners. In the top of a double boiler, combine chocolate and butter. Heat, stirring occasionally, until mixture is melted and smooth. Remove from heat and allow to cool to lukewarm.
Sift flour and sugar together into a large bowl. With mixer on low speed, beat in eggs one at a time. Stir in chocolate mixture, vanilla and pecans. Fill muffin cups 2/3 full.
Bake in the preheated oven for 25 minutes. Do not overbake. Tops should be shiny but give slightly when touched.