Step 1

Preheat oven to 350 degrees F. Line muffin pan with paper liners. Chop the bittersweet chocolate. Cut the butter into small cubes (so it melts faster).

Step 2

Set a small sauce pan on your stove with 2 inches of water and heat until barely simmering. Set a heat-proof bowl over the top, which will be used to melt the butter and chocolate.

Step 3

Add the butter, bittersweet chocolate, cocoa powder and espresso powder to the bowl over the simmering water. Heat until it's all melted and whisk until smooth.

Step 4

Set aside to cool - it should remain warm but not hot.

Step 5

Small bowl: Combine flower, baking soda and baking powder and whisk to stir it up well.

Step 6

Medium bowl: Whisk eggs briefly in the medium bowl. Then add sugar, vanilla, and salt. Whisk again until well combined.

Step 7

Add melted butter & chocolate mixture to the medium egg-sugar bowl. Whisk again!

Step 8

Sift 1/3 of flour mixture into the medium bowl, whisk until combined.

Step 9

Whisk in the sour cream. Then sift in remaining flour mixture and which until it's combined and thick.

Step 10

Pour the batter into 12 muffin cups in your muffin tin.

Step 11

Bake for about 18-20 minutes. Check for doneness by inserting a toothpick into the middle - it will come out clean when they are done.

Step 12

Get out your electric stand mixer and put on the whisk attachment.

Step 13

Whisk butter and sugar in the mixer on medium-high speed for 2 minutes. Scrape down the sides. Continue whisking on medium-high until it's light in color and very fluffy (about 2-3 more minutes).

Step 14

Turn the mixer off and add in the cocoa powder, vanilla and salt. Turn it on very low (or it will fly out of the mixer). Mix for 30 seconds, until combined.

Step 15

Add in the heavy cream, 1 tablespoon at a time, mixing for about 20-30 seconds each time to combine.

Step 16

Turn the mixer off and scrape down the sides again. This is necessary so you get a nice, even buttercream frosting.

Step 17

Turn the mixer on medium high and mix one last time, for 3-5 more minutes. It's ready when the frosting is very light and fluffy. It should be slightly shiny.