Start by preheating your oven to 175°C (350°F). While it’s heating, get your baking pan ready. You can either line it with parchment paper (make sure it covers the bottom and sides), or grease it and dust it lightly with flour so nothing sticks later.

In a medium bowl, whisk together your dry ingredients that’s the flour, cocoa powder, baking powder, baking soda, and salt. Give it a good mix so everything is evenly distributed, then set it aside for now.

Grab a large mixing bowl. Add the sugar and eggs, then beat them together you can use a hand mixer or just a whisk. You’re aiming for the mixture to turn a bit pale and get a little thick; this should take about 2 to 3 minutes. Once that’s done, pour in the oil (or melted butter if you’re using that instead) and vanilla extract. Stir until the mixture looks smooth and well blended.

Now it’s time to start combining everything. First, add about half of your dry mix into the wet ingredients. Stir gently just enough to start combining it. Then pour in the buttermilk and the hot coffee (if you’re using it it adds depth to the chocolate flavor). Finally, add in the rest of the dry mix. Stir again just until everything is smooth. Don’t overmix that can make the cake tough.

Pour the batter into your prepared pan and smooth out the top with a spatula. You can tap the pan gently on the counter to pop any big air bubbles. Then put it in the oven and bake for about 35 to 40 minutes. You’ll know it’s done when you poke the center with a toothpick and it comes out clean or with a few moist crumbs no wet batter.

When the cake’s done baking, take it out and set it on a wire rack. Let it cool in the pan for about 10 minutes. After that, you can either lift it out using the parchment or flip it out onto the rack to finish cooling completely.

Break up your chocolate bars into small pieces. Melt them gently with the butter (and a splash of cream if you want it extra rich) using a double boiler, or in the microwave in short bursts. Stir as it melts to keep it smooth don’t let it boil or get too hot, or it can seize up.

Once your cake is fully cooled, set it on a tray or a sheet of parchment. Pour or spread the melted chocolate over the top. You can let it drip down the sides for a full coat. Use a spatula to smooth it out. While the chocolate is still soft, press some chopped chocolate pieces or candy on top if you want to decorate it.

Leave the cake at room temperature until the chocolate sets. If you want it to firm up faster, stick it in the fridge for 20 to 30 minutes. Once the top is set, slice it into bars or squares and serve.