Preheat the oven to 350F. Position a rack in the center of the oven. Lightly butter two 9-inch cake pans. Line the bottoms with parchment paper, then butter and flour the parchment and sides of the pans.
Use a stand mixer with the paddle attachment on low speed to combine the sugar, flour, cocoa, salt, baking powder, and baking soda. Mix in the eggs, oil, and milk.
Increase the speed to medium and beat for 2 minutes.
Reduce the speed to low and add the boiling water. The batter should be soupy.
Divide the batter evenly between the two cake pans. Bake for 30-35 minutes, until a toothpick inserted into the center comes out clean.
Cool on a cooling rack 5 minutes. Then invert and turn out the cakes and cool completely.