Position the rack of the oven in the center and preheat to 400°F Spray the muffin pans, holes and the rims around them, with nonstick cooking spray, or fill the cups with the paper muffin cups. If using the silicone pans, spray as directed and place on a baking sheet.
Place the butter and both kinds of chocolate in the top of a double boiler set over simmering water. If you don't have a double boiler, you can use a heat safe bowl set snugly over a pan of simmering water. Stir constantly until half the butter and chocolate is melted. Remove the top of the double boiler or the bowl from the heat; then continue stirring, away from the heat, until the mixture is smooth. Cool for 10 minutes.
Meanwhile, whisk the the flour, baking soda, and salt in a small bowl until uniform.
Whisk the egg, egg yolk, and sugar in a large bowl until pale yellow and light, about 2 minutes. Slowly whisk in the chocolate mixture, continue whisking until smooth. Whisk in the sour cream until no traces of white remain. Using a wooden spoon, stir in the vanilla, then stir in the flour mixture, until moistened.
Fill the prepared pans three quarters full. Use additional greased pans or greased, oven proof for any left over batter. Or reserve for a second batch. Bake 22 minutes, or until the muffins have rounded, cracked tops and a toothpick inserted in the center or a muffin comes out with several moist crumbs attached.
Set the pan on a wire rack to cool for 10 minutes. Gently rock each muffin back and forth to release it, and cool them on the rack for 5 more minutes. If storing or freezing, cool completely before storing in airtight containers or freezer proof bags. These muffins will stay fresh for up to 2 days at room temperature, and up to 2 month in the freezer.