Heat the oven to 350 degrees. Line 24 medium muffin cups with paper lines.
In a medium bowl, stir together the flour, cocoa, baking soda and salt.
In another medium bowl, stir together 1 1/4 cups sour cream and the water.
In a large bowl, beat the sugar, oil, eggs and vanilla on medium speed for 1 minute. Alternately beat the flour mixture and sour cream mixture into the sugar mixture on low speed. Beat on medium speed for 1 minute, scraping the bowl occasionally.
Divide the batter among the muffin cups. Bake for 18 to 22 minutes or until a toothpick inserted in the center comes out clean. Remove from the pans to a wire rack. Cool for 30 minutes.
To make frosting, in a medium bowl, beat the remaining 1/3 cup sour cream and 1/4 cup butter on low speed until blended. Beat in the powdered sugar and enough milk until smooth and spreadable. Spread frosting on the cupcakes. Place a marshmallow onto the frosting. Decorate snowman faces, arms and buttons on marshmallows and cupcakes using writing frosting and candy.