3 oz. unsalted butter warm enough to be soft and pliable
4 oz. unsweetened chocolate
1 cup powdered sugar (100 grams)
2 Tbs. hot tap water
2 large eggs, room temperature
1 tsp. vanilla
Chop the chocolate into matchstick-sized pieces and melt it over a water bath, or in the microwave - however you normally do it.
Sift the sugar on top of the chocolate, without stirring. Add the hot water and stir, to combine, with a rubber spatula. The chocolate will freeze and the mixture with thicken into a paste. Don't panic, this is supposed to happen.
Add 1 egg, and stir to combine. Add the other egg, and stir. The frosting should be smooth and glossy now. If you want a smooth, dark frosting, be careful not to incorporate too much air into the mixture when adding the eggs. Personally, I usually use this recipe as a fluffy frosting, so I beat the eggs in with a whisk.
If you're squeamish about raw eggs you can heat the frosting over a water bath at this point. If you do, be sure to let it cool completely before proceeding to the next step.
Mash the butter in a medium bowl until it is smooth and creamy. Add about 1/2 of the chocolate mixture to the butter and stir (or beat if you want a fluffy frosting) until combined. Add this mixture to the remaining chocolate and stir (or beat) until smooth and homogenous (or light and fluffy).
Fill and frost your cake immediately.
Makes 1 1/2 cups.
Source of recipe: Flo Braker