Preheat the oven to 180°C (355°F)
Butter the cake pan. Line the bottom with parchment paper and then butter the top of the parchment paper. Set aside. Chop the chocolate into small pieces. Melt the chocolate in a double-boiler (we made our own) or in the microwave and let it cool. (It should be barely warm to the touch when you mix with the remaining ingredients).
In a fairly large bowl (or with an electric mixer), whisk together the butter and sugar. You'll mix until the mixture is creamy and the sugar is completely incorporated with the butter and smooth.
Mix in the eggs one at a time. You'll whisk each egg for about a minute. The mixture will become grainy which is normal.
If you're using an electric mixer, reduce the speed to the minimum. Pour in the cooled chocolate and mix until it's fully incorporated.
Keeping with the same low speed if you're using an electric mixer, add in the flour. Mix until it just disappears from the batter, but don't over-mix. You want a thick, smooth batter.
Pour the batter into the cake pan and smooth the top using a rubber spatula.
Bake the cake for 25 to 30 minutes. The top of the cake might crack a little and the cake might not look fully baked in the center which is fine.
You can test doneness of the cake by sticking a knife blade in the center - you want the knife to come out with a little bit of the batter.
Use a wire rack to cool the cake while it's still in the cake pan. Once the cake has cooled down, put it in the refrigerator for an hour or two to chill. This will make it easier to get it out of the pan.
When you're ready to take the cake out of the pan you can use two plates. Put the cake pan upside down on a plate, remove the parchment paper, and put another plate on top. Turn that over and you'll be good to go.
Let the cake return to room temperature before cutting and serving.
