Preheat oven to 350ºF. Grease 30 muffin cups (2-1/2 inch) or line with paper liners.
Whisk together flour, cocoa, sugar and baking soda in a medium bowl and set aside. Combine corn syrup, beer, butter, eggs, vanilla and sour cream in a large mixer bowl. Beat with an electric mixer for two minutes. Gradually mix dry ingredients into wet ingredients until well blended. Batter will be very thin.
Fill muffincups 2/3 full. Bake for 12 minutes; rotate pans and bake an additional 12 to 14 minutes or until wooden pick inserted in the center comes out clean. Cool five minutes; remove from pans onto wire rack. Cool completely.
Beat cream cheese in a medium bowl until light and fluffy. Beat in powdered sugar and enough cream, on low speed, to make a creamy texture. Cover with plastic wrap and refrigerate until ready to use.
Top each cupcake with a dollop of frosting and dust with cocoa powder.