Preheat the oven to 175°C.

Mix flour, cocoa powder, sugar, baking powder, baking soda and salt.

Add the eggs, buttermilk, oil and vanilla. Mix until combined.

Slowly add the hot coffee and mix until you have a smooth, fairly thin batter.

Pour into a lined rectangular baking pan and bake for approximately 25–30 minutes.

Let the cake cool completely.

Melt the dark chocolate gently and let it cool slightly.

Whip the cold heavy cream with powdered sugar and vanilla until soft peaks form.

Fold the melted chocolate gently into the whipped cream until smooth and airy.

Place the chocolate sponge on a flat board or tray.

Spread an even, thick layer of chocolate mousse over the sponge.

Smooth the surface carefully with a spatula.

Refrigerate for at least 2 hours so the mousse becomes firm.

Heat the heavy cream until hot but not boiling.

Pour it over the chopped chocolate and let it sit for 1 minute.

Stir slowly until completely smooth and glossy.

Add the butter and mix until shiny.

Pour the ganache over the chilled cake.

Spread it into a perfectly even layer.

Refrigerate for another 30–45 minutes until the ganache is set but still slightly soft.

Use a long, sharp knife.

Mark the cake into an exact grid before cutting.

Make slow, straight cuts to create identical squares.

Clean the knife with a warm, damp towel between cuts for perfectly sharp edges.

Carefully lift one square with a thin spatula.

Place it on a plate and reveal the layers: glossy chocolate ganache