Prep: Preheat oven to 350°F (175°C). Grease and line two 9-inch cake pans.
Dry Ingredients: Sift flour, cocoa powder, baking powder, baking soda, and salt into a bowl. Whisk in sugar.
Wet Ingredients: In another bowl, mix milk, oil, eggs, and vanilla until smooth.
Combine: Gradually add wet ingredients to dry ingredients, mixing on low speed. Scrape the bowl as needed.
Add Water: Carefully stir in boiling water (batter will be thin).
Bake: Divide batter evenly between pans. Bake for 25–30 minutes or until a toothpick comes out clean.
Cool: Let cakes cool in pans for 10 minutes, then transfer to a wire rack.