Preheat the oven to 350º F. Line a muffin pan with muffin liners.

Rough chop the bittersweet chocolate baking bar into smaller pieces, place in a microwave safe bowl and microwave for 2 minutes on 50% power. Stir and repeat until melted. Set aside to cool.

In a mixing bowl combine the all purpose flour, unsweetened natural cocoa powder, baking soda, baking powder and salt. Set aside.

In a separate mixing bowl beat the butter and granulated sugar with an electric mixer for 2 minutes, until pale and fluffy. Add in the eggs and cooled chocolate, beat again, 30 seconds.

Add in half of the buttermilk, beat for about 20 seconds. Add in half of the dry ingredients, beat. Add the remaining buttermilk plus the hot water, beat just until the liquid is mixed in then add the remaining dry ingredients and beat on low speed until combined.

Fill the muffin liners 1/2 full. Bake for 17-19 minutes at 350º F or until a few moist crumbs remain on a toothpick when inserted into the center. Allow the cupcakes to cool completely before adding the frosting.

In a mixing bowl, beat the butter on high for 3 minutes.

Sift in the cocoa powder and 3 cups of powdered sugar. I like to sift these in because it helps break up clumps, creating a light and fluffy frosting. Beat the frosting on low speed until combined and thick.

Mix in 3 tbsp of heavy cream, vanilla extract and salt.

Add in the additional powdered sugar (1-2 cups), mix on low. If the frosting is too thick, beat in an additional tablespoon of heavy cream.

Fit a piping bag with a piping tip, I used a Wilton 1M. Fill the bag about 3/4 full with frosting. Pipe the frosting onto the cupcakes. Decorate with chocolate curls or sprinkles if desired.