Preheat oven to 350°F. Line a muffin tin with paper liners; set aside.
In a large bowl, whisk together flour, cocoa, sugar, baking soda, baking powder and salt until thoroughly blended. Make a well in center of dry ingredients and stir in egg yolk, sour cream, 1/4 cup water and vanilla; whisk until smooth.
Spoon batter into prepared cups. Bake until a cake tester comes out clean, 20 minutes. Let pan cool completely on a rack.
Probably the easiest cupcakes I've ever made...one bowl, one whisk. Had to bake on low broil as my oven went out the minute I had to put them in. Very light and fluffy texture. Used the Vanilla Frosting from Cooking light - 1 c powdered sugar, 8 oz light cream cheese, vanilla, pinch of salt - delicious!! Will definitely make again - hopefully with oven working correctly.
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