Position a rack in the center of the oven and preheat to 350F. Lightly butter the bottom of two 9-inch round cake pans. Line the bottom with parchment paper, then butter and flour the parchment and the sides of the pans.
In the bowl of a stand mixer fitted with the paddle attachment, combine the sugar, flour, cocoa, salt, baking powder, and baking soda. Mix on low speed. Mix in the eggs, oil, and milk.
Increase the speed to medium and beat for 2 minutes. Reduce the speed to low and mix in the hot water. The batter will be soupy.
Divide the batter evenly between the cake pans. Bake for 30-35 minutes, or until a skewer inserted in the center comes out clean.
Remove from the oven and cool on a cooling rack for 5 minutes, then turn the layers out onto the rack and cool completely.
When the cakes have cooled, check the frosting. It should have the consistency of mayonnaise. If it is still too thin, allow it to cool longer.