Preheat your oven to 350°F (175°C). Line a cupcake pan with cupcake liners.
In your mixing bowl, combine the unsalted butter and semi-sweet chocolate chips. Pour the very hot coffee or hot water over the mixture and stir until melted and smooth.
Add the eggs, egg yolk, and granulated sugar to the melted chocolate mixture. Whisk until well combined, then add in the vanilla and mix again.
In a separate bowl, whisk together the all-purpose flour, baking soda, baking powder, cocoa powder, and salt. Gradually add this dry mixture into the wet ingredients, stirring until just combined.
Fold in the full-fat sour cream and coconut or canola oil until the batter is smooth.
Fill each cupcake liner about 2/3 full with batter.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Cool in the pan for about 5 minutes, then transfer to a cooling rack to cool completely before frosting.
For the frosting, beat together the softened unsalted butter and confectioners' sugar until creamy. Gradually add the cocoa powder, sour cream, half and half, salt, and vanilla. Mix until smooth and fluffy.
Once the cupcakes are completely cool, generously frost each cupcake with the chocolate frosting.
