Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.
In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In another bowl, beat the eggs, milk (or buttermilk), oil, and vanilla until smooth.
Slowly add the wet ingredients into the dry ingredients. Mix until just combined.
Stir in the hot water or coffee. The batter will be thin—this is normal.
Pour the batter evenly into the prepared pans.
Bake for 30–35 minutes, or until a toothpick comes out clean from the center.
Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
While the cakes cool, prepare the frosting by beating butter until creamy.
Add powdered sugar, cocoa powder, salt, vanilla, and cream. Beat until fluffy and spreadable.
Once the cakes are completely cool, spread frosting on the first layer, then place the second cake on top.
Frost the top and sides of the cake. Smooth with a spatula or leave swirls for decoration.
Slice and serve your classic chocolate cake.