Preheat your oven to 350°F. Grease and flour two 9-inch round cake pans or line them with parchment paper.

In a large mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt.

In another bowl, whisk together the sugar, vegetable oil, eggs, and vanilla extract until smooth and well-combined.

Add the wet mixture to the dry mixture and mix until just combined.

Stir in the buttermilk, making sure to scrape the sides of the bowl.

Gently stir in the hot water until the batter is smooth. The batter will be thin, but that's okay.

Evenly divide the batter between the prepared pans. Bake for 30-35 minutes, or until a toothpick inserted into the centre comes out clean or with a few crumbs.

Remove from the oven and let the cakes cool in the pans for about 10 minutes. Then transfer to wire racks to cool completely.

While the cakes are cooling, make the frosting. Beat the butter until smooth, then add the cocoa powder, powdered sugar, milk, and vanilla extract. Beat until fluffy.

Once the cakes are completely cool, place one layer on a serving plate and spread a layer of frosting on top. Place the second layer on top and frost the top and sides of the cake.