Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan and line it with parchment paper.
In a large bowl, whisk together flour, cocoa powder, baking powder, baking soda, salt, and sugar.
In a separate bowl, beat eggs, milk, vegetable oil, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry mixture, stirring gently.
Pour in the hot water or coffee and mix until smooth. The batter will be thin.
Pour into the prepared pan and bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool completely before slicing it into four equal layers.