Stop the motor and pour in chocolate. Mix for 1 minute, then, with the motor running, pour in the hot water. Then add the eggs, 1 egg at a time, adding the next one after the previous one has been absorbed into the mixture. Stop the motor periodically and scrape the bowl from the bottom with a rubber spatula to integrate ingredients, and return the mixer to low-medium speed. After all the eggs are added, continue to mix for an additional minute to ensure the eggs have been thoroughly mixed in. This will help guarantee that the sugar is dissolved and that the flour has been thoroughly mixed in, which will help produce a luxurious mouth-feel in the final cake.
With the motor running, add the buttermilk, stopping the motor to scrape the sides and bottom. Continue to mix for another minute or until the mixture appears smooth. Before baking, be sure the batter is at 70-73°F or the cake will crown.
Grease two 9-inch cake pans (2 inches high) with the butter, and flour them.
Divide the batter evenly between the two cake pans, using a rubber spatula to scrape down the bowl and get as much batter as possible out. Bake until the cake begins to pull from the sides of the pan and is springy to the touch, 25 to 30 minutes.
Remove the cakes from the oven and let cool for at least 30 minutes, preferably an hour. The cakes should be at room temperature before you remove them from the pan. Put a piece of parchment paper on a cookie sheet, top with sugar, and one at a time, turn the pans over and turn the cakes out onto parchment; the sugar will keep it from sticking. Refrigerate or freeze until ready to decorate.
For the Peanut Butter Frosting:
Mix soft butter and peanut butter.
Mix in sugar until it has the consistency of damp sand.
Add cream slowly until it is the thickness of your liking.
To assemble cupcakes, pour batter into a cupcake tin 3/4 of the way. Dip a quarter of a peanut butter cup in flour to prevent it from sinking to the bottom and burning and insert into the cupcake batter.