• Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Line one standard-size cupcake pan with 12 liners and an additional pan with four liners, set aside.
• Whisk together the flour, sugar, cocoa, baking soda, baking powder, and salt in a stand mixer using a low speed until combined.
• Add eggs, sour cream, buttermilk, warm water, oil, vanilla, and chopped chocolate. Beat on a medium speed until smooth, about a minute.
• Divide batter evenly between the liners (I prefer using a #20 scoop, which is approximately ¼ cup of batter).
• Place in the oven and bake for 18-22 minutes or until the tops spring back when touched.
• Remove from the oven and allow to cool in the pans for 8 minutes. Remove and transfer to wire racks to cool completely.

For the Frosting

• In a large bowl (or the bowl of a stand mixer), beat the butter and cream cheese until light and fluffy, about 2-3 minutes.
• Carefully add in the cocoa powder, vanilla, and salt, and beat until combined.
• With mixing speed on low, gradually add the powdered sugar, one cup at a time. When all of the powdered sugar has been added, beat in heavy cream, then add more if necessary to create a spreadable consistency. The chocolate frosting should be on the thicker side.
• Using a piping bag fitted with an extra-large piping tip (see NOTES below for suggestions), pipe onto cooled cupcakes and sprinkle with the desired garnish.