For the cake:

In a bowl combine the flour, cocoa, sugar, baking soda and salt. Set aside.

In the bowl of an electric mixer, combine the butter, sour cream, milk, egg and vanilla extract. Add the dry flour mixture a spoonful at a time into the wet ingredients. Stop the mixer between additions to scrape down the sides of the bowl to ensure all the ingredients are incorporated.

Preheat your oven to 375°F (190°C).

Line a muffin tin with paper liners and fill each generously but evenly with the batter. Bake for 20-25 minutes until a toothpick comes out clean from the center. Remove from the oven and allow to cool completely before frosting with Four Minute Frosting.

Frosting:

Fill the bottom part of the double boiler with water and bring to a boil. Make sure the water level is just below the bottom of the top pan.

Combine the egg white, sugar, water, corn syrup and salt in the top pan of the double boiler. Using an electric hand mixer, beat the ingredients until just combined, about 20 seconds. Place the pan over the boiling water and beat with the electric mixer for 4 full minutes. The mixture should form stiff peaks. Remove the pan from the boiling water, add the vanilla, and continue to beat for 2 more minutes.

Allow to cool briefly before frosting the cupcakes.