Preheat your oven to 175°C (350°F). Grease and flour two 23cm (9-inch) round cake pans. You can also line the bottoms with parchment paper for easier removal. This ensures the cake layers won't stick.

In a large mixing bowl, whisk together the sifted all-purpose flour (300g or 2 ½ cups), granulated sugar (400g or 2 cups), unsweetened cocoa powder (75g or ¾ cup), baking soda (1 ½ tsp or 7.5g), baking powder (1 tsp or 4g), and salt (1 tsp or 6g). Ensure all dry ingredients are well combined and there are no lumps of cocoa powder. This step is crucial for an even rise and texture.

In a separate medium bowl, whisk together the large eggs (2), buttermilk (240ml or 1 cup), vegetable oil (120ml or ½ cup), and vanilla extract (2 tsp or 10ml). Whisk until thoroughly combined.

Pour the wet ingredients into the large bowl with the dry ingredients. Beat on medium speed with an electric mixer (or whisk vigorously by hand) for about 2 minutes, until just combined. Be careful not to overmix at this stage.

Carefully add the hot water or hot brewed coffee (240ml or 1 cup) to the batter. The batter will be thin – this is normal and contributes to the cake's moistness. Mix on low speed until just incorporated. The hot liquid helps to bloom the cocoa powder, intensifying the chocolate flavor.

Divide the batter evenly between the two prepared cake pans. You can weigh them to ensure equal amounts for even baking. Tap the pans gently on the counter a couple of times to release any large air bubbles.

Bake in the preheated oven at 175°C (350°F) for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cakes comes out clean. Start checking for doneness around the 30-minute mark to avoid overbaking.

Remove the cakes from the oven and let them cool in the pans on a wire rack for about 10-15 minutes. This allows the cakes to firm up slightly before being inverted.

Carefully invert the cakes onto the wire rack to cool completely. Ensure they are fully cooled before frosting; otherwise, the frosting will melt.

While the cakes are cooling, prepare the chocolate frosting. In a large bowl, beat the softened unsalted butter (230g or 1 cup) with an electric mixer until creamy and smooth.

Gradually add the sifted powdered sugar (450g or 3 ½ cups) and unsweetened cocoa powder (60g or ⅔ cup) to the butter, alternating with the milk or heavy cream (60ml or ¼ cup). Start and end with the dry ingredients. Beat on low speed until just combined, then increase to medium-high speed and beat for 2-3 minutes until light and fluffy.

Beat in the vanilla extract (1 tsp or 5ml). If the frosting is too thick, add more milk or cream, 1 teaspoon at a time, until it reaches your desired spreading consistency. If it's too thin, add a little more sifted powdered sugar.

Once the cake layers are completely cool, place one layer on your serving plate or cake stand. Spread a generous layer of frosting evenly over the top.

Carefully place the second cake layer on top of the frosted first layer. Gently press down to secure it.

Frost the top and sides of the entire cake with the remaining chocolate frosting. You can create smooth sides or decorative swirls using an offset spatula or the back of a spoon.

Slice and serve immediately, or let the frosting set slightly at room temperature for about 30 minutes before slicing. The cake can be stored at room temperature in an airtight container for up to 3 days.