Adjust oven rack to middle position and heat oven to 350 degrees F. Line standard muffin tins with paper liners; set aside.

Place cake mix, sour cream, melted butter, eggs and vanilla extract in a large bowl. Blend with an electric mixer on low speed until dry ingredients are moistened, about 30 seconds. Increase mixer speed to medium and beat until smooth, about 3 to 4 minutes, stopping to scrape bowl as needed. The batter should be very thick and creamy.

Coat paper liners in muffin tins with cooking spray. Spoon batter evenly into baking cups, filling each two-thirds full. Level batter and release any air bubbles by tapping muffin pan on counter top. (You should get between 22 and 24 cupcakes; remove any empty liners.)

Place pans in oven and bake for 20 to 25 minutes, or until a toothpick inserted in center comes out clean. (When done, cupcakes should spring back when lightly touched in center.) Remove pans from oven and place on wire racks to cool for 10 minutes. Gently remove cupcakes from pans and place them on wire racks to cool completely before frosting.

Meanwhile, prepare Chocolate Buttercream. Combine butter and cream cheese in a large bowl and beat at medium speed with an electric mixer until creamy, stopping to scrape bowl as needed. Gradually add confectioners' sugar, beating at low speed until blended. Increase speed to medium, and slowly add milk and vanilla, beating until smooth.

Microwave dark chocolate in a microwave-safe bowl at 50% power for 1 1/2 to 2 minutes or until just melted and smooth, stirring at 30-second intervals (see note). Alternately, the chocolate may be melted using the double-boiler method. Gradually add melted chocolate, a spoonful at a time, to butter mixture; beating until completely incorporated and smooth. (Makes about 3 cups) When chocolate cupcakes are completely cool, frost the tops with Chocolate Buttercream.

Makes about 2 dozen.

Cooks Note for Dark Chocolate:

I use Nestle Chocolatier (53% Cocoa) Dark Chocolate Morsels, Dark Chocolate Chunks, or Dark Chocolate Baking Bars.

Cooks Notes for Melting Chocolate:

If chocolate is heated too much, the cocoa butter can separate from the cocoa solids, streaking the chocolate gray. Chocolate must be heated without a drop of moisture reaching it and never to more than 115 degrees F (for dark chocolate). If you overheat chocolate, it scorches and becomes grainy, as it also will if even a drop of moisture falls into the melting mass.

When melting chocolate in the microwave, it can retain its shape, which can fool you into thinking it's not melting. Be sure to stir it between heating intervals so you can gauge if it has melted, otherwise you might overheat it.

For the double-boiler method, place chocolate in a stainless-steel bowl or top of a double boiler, and set bowl over a saucepan of hot or gently simmering (not boiling) water. Do not allow bottom of bowl to come in direct contact with hot water, nor cover bowl or double boiler. When chocolate begins to melt, stir chocolate from sides of bowl into the center mass. When chocolate is almost entirely melted, remove bowl from heat, stir, and let remaining chocolate melt from heat of the larger melted mass.