Preheat the oven to 350 degrees F.

Measure all of the ingredients into a large bowl in the order that they are listed. Beat the mixture until smooth (about 30 seconds to no more than one minute), scraping the sides as needed.

Line a cupcake pan with cupcake wrappers. Fill the cupcake wrappers 3/4 full.

Bake for 16-17 minutes, or until the cupcake bounces back when you touch the top.

Remove the cupcakes to a wire rack to cool completely before frosting.

Melt the unsweetened chocolate and butter in a double boiler. (Place 1 inch of water in the bottom pan and the butter/chocolate in the top pan. Heat the water on low and stir the butter/chocolate mixture until smooth.)

Set the chocolate aside for 5 minutes to allow it to cool slightly.

Once the chocolate has cooled slightly, use a whisk to stir the sour cream and vanilla into the chocolate.

Add the powdered sugar and use a hand mixer to mix the frosting until it is well combined and smooth.

Cut 1" of the tip off of a disposable decorating bag.

Fill the decorating bag with frosting.

Hold the decorating bag perpendicular to the cupcake, not quite touching the top.

Squeeze the bag firmly and gently make a circle to fill the top of the cupcake with frosting.

(You can also just spread the cupcake with a knife.)

Repeat with the remaining cupcakes.

Store in the refrigerator in an airtight container.