Preheat the oven to 350℉ (177℃).
Cream sugar and butter. Add eggs one at a time, mixing well after each addition and stopping to scrape the bottom and sides of the bowl.
2 cups (400g) granulated sugar, 1 cup (227g) salted butter, 5 large eggs
Add the flour, baking powder, and salt alternately with the milk, vanilla, and water.
3 cups (423g) all-purpose flour, 1/2 teaspoon salt, 1 1/2 teaspoons baking powder, 1 cup evaporated milk, 2 teaspoons pure vanilla extract, 1/2 cup water
Spray 9-inch cake pans with baking spray. Evenly divide the batter to get 8-10 layers. I used 3/4 cup of batter per layer (it was 187 grams of batter per layer, to be precise) and ended up with 9 layers. Use the back of a spoon to spread the batter into thin, evenly layers.
Bake the layers for 8-10 minutes until done. With cake layers this thin I find the best way to check whether the layer is done is to listen to it. If you still hear it making a spitting or sizzling noise, let it bake for another minute or two until it is quiet. You can also gently tap the top of the layer with a finger. If it leaves an indentation then the cake layer needs to bake longer.
Make the icing while the first layers are baking. As each layer finishes, let it cool for a few minutes before turning it out onto a cake plate, topping with 2-3 spoonfuls of icing, and continuing to bake, stack, and ice.
To make the icing, combine the sugar and evaporated milk in a medium saucepan over medium-low heat for 3-4 minutes until warm. Add the chocolate and stir until melted and combined. Add the butter and continue to cook until melted.
2 cups (400g) granulated sugar, 1 cup evaporated milk, 5 ounces unsweetened chocolate, 1/2 cup salted butter
Increase the heat to medium-heat and continue to cook at a low boil or simmer, stirring frequently for 10-15 minutes until thickened slightly. Remove from the heat and stir in the vanilla. The icing will still be very thin but will continue to thicken as it cools.
1 teaspoon pure vanilla extract