Source  Brown Eyed Baker

Recipe Added:  4/21/2012 

Preheat oven to 350F. Line 2 cupcake pans with paper liners. Combine cocoa powder and hot water in a cup and whisk until smooth. In a medium bowl whisk flour, baking soda, baking powder, and salt. In saucepan over medium heat, combine butter and sugar. Cook, stirring occasionally, until mixture is smooth and butter is melted. Transfer to a mixer bowl and beat on medium-low speed until the mixture is cool, about 4-5 minutes.

Add eggs one at a time, mixing well after each addition. Blend in vanilla and cocoa mixture until smooth. On low speed, add flour mixture in three additions alternating with sour cream. Divide batter evenly between muffin liners, filling 2/3 full. Bake one pan at a time, 15-18 minutes. Remove to a wire rack to cool completely.

Buttercream Frosting Using the wire whisk attachment of a stand mixer, whip butter on medium-high speed 5 minutes. Reduce the speed to low and gradually add powdered sugar. Increase speed to medium-high and add vanilla. Add melted chocolate and whip at medium-high speed until light and fluffy, about 2 minutes, scraping the bowl as needed. Store unused frosting in the refrigerator in an airtight container; let it come to room temperature and then give it a quick whip in the mixer before using.

Alternate Frosting Version In a food processor, process the butter, sugar, cocoa powder and salt until smooth, about 30 seconds. Add corn syrup and vanilla and process until just combined, 5 to 10 seconds. Scrape sides of bowl, then add chocolate and pulse until smooth and creamy, 10 to 15 one-second pulses. Frost the cupcakes and store in airtight container room temperature for up to 3 days.

1. http://www.browneyedbaker.com/2013/08/19/best-chocolate-cupcakes-recipe/

2. http://www.browneyedbaker.com/2010/10/22/best-chocolate-buttercream-cupcakes/

3. http://www.melskitchencafe.com/2012/04/perfect-chocolate-cupcakes.html