Preheat oven to 300°F (148°C).
Add the flour, sugar, black cocoa powder, baking soda, and salt to a large mixing bowl and combine. Set aside.
Add the egg, milk, vegetable oil, and vanilla extract to another medium-sized bowl and whisk together to combine.
Add the wet ingredients to the dry ingredients and mix until well combined.
Add the water to the batter and mix until well combined. The batter will be very thin.
Fill the silicone cake mold about 3/4 way and bake for 25-30 minutes, or until a toothpick comes out with a few moist crumbs.
Remove the cakes from the oven and allow them to cool then remove to a cooling rack to finish cooling.
Once cooled, make three holes in even spacing on the bottom of the cakes. Through these holes, you will pipe the frosting later. Set them aside.