Preheat oven to 350°F (175°C).

Grease or line 24-30 muffin cups.

In a heavy saucepan, melt butter with Guinness and cocoa powder over low heat, stirring until smooth.

Remove from heat and let cool slightly.

In a large bowl, sift together salt, flour, sugar, and baking soda.

Add the Guinness cocoa mixture to the dry ingredients and beat on medium speed for 1 minute.

Add eggs and sour cream, and beat for 2 minutes on medium speed.

Divide batter evenly among muffin cups.

Bake for 20-25 minutes, or until a wooden pick inserted in the center comes out clean.

While cupcakes bake, prepare the frosting.

In a medium bowl, beat cream cheese, butter, confectioners' sugar, milk, and vanilla until smooth and creamy.

Add green food coloring for a festive look (optional).

Remove cupcakes from oven and let cool in the muffin tin for 5 minutes.

Transfer to a wire rack to cool completely.

Frost cupcakes as desired.

Sprinkle with green sprinkles or sugar (optional).