Preheat the oven to 160 degrees Celsius. Line 18-20 cupcake moulds with cupcake liners.
Sift the flour, cocoa powder, baking soda, baking powder and salt into a medium bowl. Set aside.
In a separate large bowl, use an electric mixer and beat the butter on medium speed until creamy for 30 seconds. Add in 1/3 of the the sugars and beat on medium speed for 1 minute. Scrape the sides down with a rubber spatula. Repeat for another two times. Scrape the sides and beat on high speed for 2 minutes until the mixture is creamy and fluffy.
Add in the egg, beat on low speed for 30 seconds then beat on medium speed for 1 minute. Scrape the sides.
Add in the buttermilk and vanilla extract. Similarly, beat on low speed for 30 seconds then medium speed for 1 minute. Scrape the sides.
Add in half of the flour and fold gently with a large rubber spatula or a metal spoon. Fold in the remainig flour gently.
Divide the batter among the cupcake moulds and bake for 20 - 25 minutes, until well-risen and a skewer inserted into the center of the cake comes out clean.
Let the cupcakes cool on a rack completely.
Frosting:
In a medium bowl, use an electric mixer and beat the icing sugar, butter and peanut butter until smooth and creamy. Scoop the frosting into a piping bag.
Pipe the frosting onto the cupcake and decorate with some Varlhona crunchy chocolate pearls.
NOTES
Dutch-processed cocoa powder (also known as alkalized unsweetened cocoa powder) is natural cocoa powder being treated with alkali so it is not acidic. Losing its acidity, it will require the addition of baking powder for the baked goods to rise. Natural cocoa powder (such as Hershey's) is acidic hence if you are using it, omit the baking powder and increase the amount of baking soda used to 1/2 teaspoon. (The end product will be lighter in colour if using natural cocoa powder.)
Original amount of sugar that is called for: 200 grams light brown sugar and 100 grams caster sugar. This batch of cupcakes tasted ok to me - it may not be sweet enough if having it on its own, but the sweetness of the frosting made it up.
To make your own buttermilk: combine 15 ml vinegar (1 tablespoon) with 235 ml milk and stir gently. Leave it aside for 15 minutes and it's ready to be used. I've always been making my buttermilk this way.
80 grams of icing sugar is not a lot - hence the frosting is a bit soft and not very sweet (which was how I liked it). Do add more icing sugar if you like and taste as you go (:
I didn't pipe a lot of frosting on the cupcakes, hence I only used 2/3 of the frosting, but I really think these cupcakes deserve more frosting! I transferred the remaining 1/3 of the frosting into a small bowl and serve it together with the cupcakes. My friends who wanted more frosting can scoop more for themselves (: