Ganache

1. Finely chop the chocolate and place it in a heatproof bowl.Heat 270 ml of the cream in a saucepan over medium heat until it just begins to simmer around the edges without boiling.
2. Pour the hot cream over the chocolate and let sit for 1 to 2 minutes.
3. Stir slowly from the center outward until the chocolate is fully melted and smooth.
4. Add the remaining 270 ml cold cream and mix until completely smooth.
5. Cover with plastic wrap pressed directly onto the surface and refrigerate for at least 6 hours or overnight until very cold and set.

Chocolate Cake

1. Preheat the oven to 165°C. Line the bottom of a 9x13-inch (23x33 cm) pan with parchment paper and lightly grease the sides.
2. In a large bowl, whisk together the sugar, oil, eggs, and vanilla for 1 to 2 minutes until smooth and slightly thickened.
3. In another bowl, sift together the cake flour, cocoa powder, baking soda, baking powder, and salt. In a jug, mix the buttermilk with the warm coffee or warm water.
4. Add one third of the dry ingredients to the wet mixture and whisk gently. Add one third of the buttermilk mixture and whisk again. Continue alternating dry ingredients and buttermilk until everything is incorporated. Mix just until smooth with no visible streaks of flour. The batter will be fairly fluid.
5. Pour the batter into the prepared pan and gently tap the pan on the counter to release air bubbles.
6. Bake for 35 to 40 minutes, until the cake springs back lightly and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.
7. Let the cake cool completely in the pan on a wire rack before frosting.

Assembly

1. Place the fully chilled ganache in the bowl of a stand mixer fitted with the whisk attachment, or use a hand mixer.
2. Whip on medium speed until the ganache becomes lighter in color, thick, and holds medium to firm peaks. Stop as soon as it is fluffy and spreadable to avoid overwhipping.
3. Spread the whipped ganache evenly over the completely cooled chocolate sheet cake, going all the way to the edges. Finish with chocolate sprinkles and cut into about 12 squares or more before serving.