• Preheat the oven to 350 F (180 C) for conventional oven. For a convection oven, lower the temperature by 20 F (15 C)
• Line the muffin pan with cupcake liners

Prepare the cake batter:

• 30 minutes before you plan to bake, get the egg, sour cream (or yogurt) out of the fridge to let them come to room temperature
• In a large mixing bowl, add flour, cocoa powder, baking soda, baking powder, and salt. Whisk to combine and set aside
  
• Add oil, sugar, egg, vanilla, and sour cream in another mixing bowl or large measuring cup. Whisk until the mixture is emulsified
  
• Add the egg mixture into the flour mixture. Whisk to combine. It may seem really thick at this point. Add the hot boiling water and continue whisking until everything is well-combined. The batter is semi-thick but pourable consistency. I like to transfer it to a large measuring cup with a spout for easy pouring
  
• Fill the muffin pan about 3/4 full
  

Baking:

• Place the muffin pan in the middle rack and bake for 19-20 minutes or until a toothpick inserted into the center of the cupcake comes out clean with a bit of sticky crumbs is okay, but it shouldn't be wet. If it is, bake another 2-3 minutes and check again

Cooling:

• Let the cupcake cool in the pan for 5 minutes. Don't try to remove them as soon as they are out of the oven. They are still too tender and may break apart
  
• Then remove to the cooling rack to cool completely
  

Prepare sour cream frosting:

• Prepare a double boiler. I usually fill a saucepan with water and then set a heat-proof bowl on top of the saucepan, not touching the water. Melt the chocolate and butter over the lowest heat until only a few chips are still not melted. Remove from the heat, add the vanilla extract and continue to stir until all chocolate has melted.
  
• In a mixing bowl, add powdered sugar, cocoa powder and then the melted chocolate. Stir the mixture a little bit so that the powdered sugar won't "fly" all over when you start mixing. Use a handmixer or a standmixer with a flat beater and beat the mixture on the lowest speed first, then increase to medium speed and beat for about 30 seconds.
  
• The mixture is very stiff and clumpy at this point.
  
• Add the cold sour cream and continue to whip over medium speed
  
• It will be lumpy at first but will smooth up and start to look shiny. Continue to whip
  
• You need to whip until it is light in color, smooth, and fluffy
  

How to store frosting for future use:

• If not using it anytime soon, cover and keep it in the fridge for 3-4 days. It can be kept frozen for up to 1 month
• If it's been in the fridge, let it sit at room temperature for about 30 minutes and then beat again until it is smooth and shiny again

Decorating:

• Transfer the frosting to a piping bag fitted with a decorating tip of your choice and start piping on top of the cupcakes. It generously frosted 12 cupcakes. You may have a bit of leftover depending on how much you pipe
  
• The frosting is very stable and pipe with sharp edges. Scatter some mini chocolate chips on the frosting if you wish
  

How to store:

• Store the frosted cupcakes in an air-tight container and keep in the fridge for 3-4 days. For longer storage, store them in the freezer for up to 2 months. Simply thaw at room temperature for a few minutes before serving