Preheat the oven to 350ºF. Coat three 8 inch cake pans with shortening and dust with flour. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine sugar and shortening until light and fluffy. Add the eggs.
Add vanilla extract to the milk. In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.
Add the flour alternately with the milk, pulsing the mixer to avoid ingredients splashing out. You will start with flour and end with flour, adding flour four times and milk three times. Add the boiling water all at once and pulse until combined.
Divide the batter between the three pans, smooth it out, and bake until a toothpick inserted in the center comes out clean, about 20 minutes. Move to a cooling rack to cool.
To make the butter sauce, combine all ingredients in a medium saucepan and melt, stirring, over medium heat. Heat until hot but not boiling. Pour the hot sauce over slices of chocolate cake and serve.