In a saucepan over low heat, melt the butter and chocolate chips, stirring until smooth. Cool. Add pecans; stir until well-coated.
In a large bowl, combine eggs, sugar, flour and vanilla. Fold in chocolate mixture. Fill paper-lined muffin cups two-thirds full. Bake at 325 degrees for 35-38 minutes or until a toothpick inserted near the center comes out clean.