For the cake. Preheat oven to 350°F. Spray four 9-inch round cake pans with nonstick baking spray and line with parchment paper circles.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, beat at medium-high speed, the sugar and butter until light and fluffy. About 4 minutes. Add the eggs, one at a time and beat until well incorporated. Scrape the sides of the bowl as needed. Reduce the speed to low and add the flour mixture. Add milk, water, and vanilla. Mix until combined.
Evenly distribute half of the batter into each of the four prepared pans. Using an offset spatula spread the batter.
Bake for 10-12 minutes until lightly browned around the edges. Remove from the oven and carefully transfer to a cooling rack. Allow the cake pans to cool down and prepare with spray and parchment paper. Repeat the process with the rest of the batter. You will have eight total layers of cake. Allow the layers to cool completely.
While the cake layers are cooling down, prepare the chocolate icing. In a large saucepan, combine sugar and milk and cook over medium heat until the sugar dissolves. Add the chocolate and butter and stir until well combined. Cook at medium-low heat for 10-15 minutes, stirring occasionally. Remove from heat and add vanilla. Use immediately.
Assemble the cake. Place one layer of cake on a stand or serving platter. Spread about ¼ cup of icing. Repeat the process with the rest of the cake layers. Spread icing over the top and sides of the cake. Allow the cake to rest and the icing to harden before cutting and serving.