Preheat your oven to 350°F (175°C) and prepare two 8 or 9-inch round cake pans by greasing, flouring, or lining with parchment paper.

In a large bowl, thoroughly whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt.

In a separate medium bowl, combine the eggs, milk, vegetable oil, and vanilla extract by whisking them together.

Pour the wet ingredients into the dry ingredients and mix until just combined, then gradually add hot water or coffee, mixing until the batter is smooth.

Divide the thin batter evenly between the prepared cake pans and bake for 30-35 minutes, or until a wooden skewer comes out clean.

Allow the cakes to cool in the pans for 10-15 minutes before transferring them to a wire rack to cool completely.

While the cakes cool, beat the softened butter until creamy, then gradually add sifted powdered sugar, cocoa powder, and a pinch of salt, alternating with milk until smooth and fluffy, finally beating in the vanilla extract.

Once the cakes are completely cool, frost and decorate them with the prepared chocolate buttercream.