Preheat your oven to 350 degrees and prepare a 9×13-inch baking pan by spraying it with baking spray that contains flour. This ensures your cake won’t stick.
In a medium bowl, place the chopped unsweetened chocolate. Pour the boiling water over it and let it stand until the chocolate is completely melted, stirring a few times. Once melted, allow it to cool to tepid.
On a piece of waxed paper, sift together the cake flour, baking soda, and salt. This step helps to aerate the flour and ensures an even distribution of the leavening agent.
Using an electric stand mixer, combine the butter and shortening, beating on medium speed for about 3 minutes until it's light and fluffy.
Gradually add the superfine sugar in three additions, beating for 1 minute after each addition. This method helps to incorporate air into the mixture, leading to a lighter cake.
Now, add the eggs one at a time, beating for 45 seconds after each addition. You may need to scrape down the sides of the bowl a few times to ensure everything is well mixed.
Slowly incorporate the melted chocolate and vanilla extract, mixing until well combined.
On a low speed, alternate adding the sifted flour mixture and the buttermilk in three additions of flour and two of buttermilk. This method helps maintain the cake's moisture and texture.
Once your batter is ready, transfer it to the prepared baking dish and smooth out the top with a spatula. Bake for 35 to 40 minutes or until a toothpick inserted in the center comes out clean.
After baking, allow the cake to cool completely before frosting. This is crucial; frosting a warm cake can cause it to melt.
To prepare the frosting, in an electric mixer, combine half of the confectioners' sugar and salt in a bowl.
In a separate bowl, whisk together the cooled melted chocolate and butter until blended. Transfer this mixture to the bowl with the confectioners' sugar and beat on LOW speed for 1 minute.
Next, add the milk and vanilla extract, beating for 2 minutes on medium speed. Remember to scrape down the sides of the bowl to ensure an even mix.
Incorporate the remaining confectioners' sugar, mixing in well. Then, add the heavy cream and beat on medium-high speed for 2 minutes. If the frosting seems too thick, feel free to add a bit more cream.
Finally, spread the luscious frosting over the cooled cake, and it's ready to serve!