Preheat your oven to 350°F (175°C). Line your muffin tin with cupcake liners.
In a large mixing bowl, whisk together the eggs, sour cream, vegetable oil, and vanilla extract until smooth.
Add the cake mix and instant pudding mix to the wet ingredients. Stir until just combined.
Pour in the hot water or coffee and mix until fully incorporated.
Fold in the mini chocolate chips or chopped chocolate bar.
Divide the batter evenly among the lined cupcake wells, filling each about 2/3 full. Bake for 18-20 minutes.
Once baked, cool the cupcakes in the tin for 5 minutes, then transfer to a cooling rack.
In a medium bowl, beat together the butter, powdered sugar, cocoa powder, heavy cream, vanilla extract, and salt until creamy.
Once the cupcakes are cool, frost them generously with the chocolate frosting.