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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Place cupcake liners in two (12-well) muffin pans; alternatively, coat the wells with melted butter.
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3
Set the pans aside.
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4
Whisk together the Guinness, milk, vegetable oil, and vanilla in a large bowl until combined.
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5
Whisk in the eggs, one at a time, until completely incorporated.
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6
Whisk in the sour cream.
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7
Whisk the flour, sugar, cocoa powder, baking soda, and salt in a second large bowl to aerate and combine.
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8
Gradually fold this mixture into the wet Guinness mixture.
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9
Divide the batter among the muffin wells.
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10
Bake until a toothpick inserted into the center of the muffins comes out clean, about 20 to 25 minutes.
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11
Set the pans on a wire rack and let the cupcakes cool completely before removing from the pans.
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12
Place the cream cheese and salt in the bowl of a stand mixer fitted with a paddle attachment and beat on medium speed until light and fluffy, about 2 minutes.
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13
With the mixer on medium speed, gradually beat in the heavy cream until smooth, about 1 minute.
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14
Scrape down the sides of the bowl with a spatula.
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15
With the mixer on medium speed, gradually mix in the powdered sugar until completely incorporated and smooth, about 3 minutes.
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16
Top each cupcake with a heap of frosting and dust with cocoa powder.
