Cost per Serving:
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.
Frost with buttercream and decorate as desired.