For the cupcakes:
Unsalted butter, melted, for coating the muffin pans (optional)
1 (11.2-ounce) bottle Guinness stout
1/2 cup whole milk
1/2 cup vegetable oil
1 tablespoon vanilla extract
3 large eggs
3/4 cup sour cream
2 1/2 cups all-purpose flour
2 cups granulated sugar
3/4 cup natural unsweetened cocoa powder
1 1/2 teaspoons baking soda
1/2 teaspoon fine salt

For the frosting:
1 (8-ounce) package cream cheese, at room temperature
Pinch fine salt
1/3 cup heavy cream
1 pound powdered sugar
Natural unsweetened cocoa powder, for dusting