For the cupcakes:

1. Preheat the oven to 350°F (178°C).

2. In a medium bowl, combine the chocolate and cocoa powder and pour boiling water over the mixture. Whisk mixture gently until smooth then set aside to cool.

3. In a small bowl whisk all dry ingredients.

4. Whisk yogurt, discard, oil, eggs, egg yolk, vinegar, and vanilla into the cooled chocolate mixture.

6. Stir in the flour mixture until smooth (do not overmix).

7. Divide the batter into the muffin tin with paper liners.

8. Bake at 350°F (178°C) for 20-25 minutes, until a toothpick comes out clean when inserted.

10. Allow the cupcakes to cool for 2-3 minutes before removing carefully from the pan and placing onto a cooling rack.

For the buttercream:

1. In a small saucepan, bring the cream to a simmer. Remove from heat and add the chocolate.

2. Let stand for a few minutes; then stir until smooth. Set aside to cool.

3. In the bowl of a stand mixer with the paddle attachment, beat the butter on medium speed until creamy, add icing sugar, cocoa powder, vanilla, and salt. Mix on low speed until well-belended. Increase the speed to medium and beat for another 3 minutes.

4. Add the cooled chocolate cream mixture. Increase the speed to medium-low and beat until fluffy and spreadable.

5. Fill a piping bag with the buttercream and pipe onto cooled cupcakes. You can also use a butterknife. Top with sprinkles if desired.