Preheat the oven to 350°F (180°C). Grease a 9-inch round cake pan and dust with flour or cocoa powder; you can also line the bottom with parchment paper.
In a large bowl, whisk together the flour, granulated sugar, unsweetened cocoa powder, baking powder, and salt until well combined.
In a separate bowl, whisk the eggs, milk, vegetable oil, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently whisk until just combined.
Stir in the hot water until the batter is smooth and thin—that’s normal.
Pour the batter into the prepared pan and bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake rest in the pan for 10 minutes. Then carefully remove it and transfer to a wire rack to cool completely before slicing.