In a medium, heavy-bottomed saucepan, sift together the sugar, flour, and cocoa powder. Whisk in the salt and milk.

Heat over medium heat, stirring continuously. The mixture will begin to thicken after 8-10 minutes. Continue cooking and whisking for an additional 2 minutes, then remove from the heat and stir in the vanilla extract. Scoop the mixture into a medium bowl and using a silicone spatula, stir occasionally as it cools to room temperature, about 30 minutes. Then place plastic wrap directly on the surface of the mixture and place in the refrigerator to completely cool and set for 2 hours.

Meanwhile, preheat the oven to 350°F and place racks on the two middle positions. Spray the bottom and sides 3 9-inch rounds with nonstick baking spray.

Add eggs and sugar to the bowl of your stand mixer and beat on high speed for 2-3 minutes.

While the mixture beats, add cocoa powder and instant coffee to the hot water in a separate bowl. Slowly add this mixture to the eggs and sugar.

Sift flour, baking soda, baking powder, and salt in a small bowl. Reduce the speed of the mixer to low then add in the dry ingredients in two intervals. Next, add the vegetable oil, buttermilk, and vanilla extract and beat until just combined. The batter will be sort of thin, but this is normal.

Evenly divide the cake batter into all three pans and bake for 22-25 minutes, or until an inserted toothpick comes out clean. Halfway through baking rotate the pans from front to back and revolve from top to bottom.

Remove the cakes from the oven and allow to rest for 10 minutes before removing from the pans and placing on a wire rack to cool completely, about 30 minutes.

After the pudding mixture has cooled, melt the chocolate in a medium microwave-safe bowl and heat on low for 3 minutes, stirring at 20 second intervals until completely melted. Let sit for 5 minutes to cool.

Add the softened butter to the bowl of your stand mixer fitted with the whisk attachment and whip the butter for about 1 minute, until light and smooth.

Add the pudding mixture in three intervals, mixing on medium and stopping to scrape down the sides and especially the bottom of the bowl to ensure everything is incorporated. Reduce the speed and slowly pour in the cooled chocolate and salt. Increase the speed to high and mix for an additional minute, scraping down the sides and bottom of the bowl. The frosting should look rich and have a dull sheen.

Place the bottom layer on your cake stand. Place some of the frosting on top and spread evenly just over the edges of the cake. Repeat with the next layer of cake and frosting.

Finally, place the last layer of cake on top, then cover the top and sides of the cake with a very thin layer of frosting and smooth it to make it even. This will be your crumb coating to make it easier for the final layer of frosting. Chill in the refrigerator for 30 minutes to allow the layers and frosting to set. Remove the cake from the refrigerator and coat the top and sides with the remaining frosting and smooth it. Slice and serve immediately.