Heat the oven to 355°F (180°C). Line two 12-cup muffin pans with 18 paper liners total.
Bloom the cocoa: In a heatproof bowl, whisk the cocoa powder with the hot coffee until smooth and glossy. Set aside to cool for 5 minutes.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon until evenly combined.
In a separate bowl, whisk the eggs, egg yolk, oil, vanilla, and sour cream until silky. Stream in the cooled cocoa-coffee mixture and whisk until uniform.
Add the dry ingredients to the wet in two additions, folding gently with a spatula just until no dry streaks remain. Scrape the bowl, then let the batter rest for 5 minutes to hydrate.
Portion the batter into liners, filling each about 3/4 full. Bake 13–15 minutes, rotating the pans halfway, until a tester comes out with a few moist crumbs. Cool in the pan for 10 minutes, then transfer to a rack to cool completely.
Make the frosting: In a stand mixer (paddle) beat the butter, cocoa, vanilla, and salt on medium-high for 2 minutes until fluffy and dark. Mix in 2 cups of powdered sugar on low. Add 2 tablespoons cream and beat smooth.
Add the cream cheese in chunks and beat on medium just until creamy and lump-free, 1–2 minutes. Add the remaining powdered sugar and enough cream to reach a soft, pipeable consistency; beat 1–2 minutes. If warm, chill the frosting for 10 minutes before piping.
Transfer frosting to a large piping bag fitted with an open star tip and swirl generously onto cooled cupcakes.