Preheat the oven to 350 degrees.
Prepare your cake pan or pans. Butter the sides of the cake pan, and then dust with flour. Then place a piece of round parchment in the bottom of the pan. Set aside. You have enough batter for a 9" cake pan, but use any smaller size if preferred. This cake freezes well also, so you may make multiple small cakes.
Over low heat in a medium or small sauce pan, melt the chocolate and butter. Mix while it melts. Add the salt while mixing to incorporate all three ingredients. Once the ingredients are one lovely chocolate color, remove from the heat to cool to warm.
In a mixing bowl, mix the eggs (the fresher the eggs, the shorter the time needed to whisk into a lovely frothy, airy mixture). Add the sugar and then whisk for about 2-4 minutes to reach a lovely frothy, foamy consistency. Use a hand mixer.
Making sure the chocolate mixture has cooled down to warm from hot, add about 1/3 of the egg mixture to the chocolate and fold in gently with a spatula to keep as much of the air as possible. Then add another third, fold, and then add another fold until completely combined, but don't over-mix.
Pour the batter into the cake pan and bake for 30-40 minutes (30 for a 6" pan, 40 for a 8 or 9" pan). Check at 25 minutes by placing a toothpick or knife in the middle. You don't want the cake to be entirely baked through as you want a bit of fudgie consistency in the middle. If the the latter third or half of the toothpick is still a bit gooey, the cake is done. Once you remove the cake from the oven, prepare to remove from the pan. Make sure all of the sides are not sticking (use a knife and run it along the inside of the pan all the way around). Place a plate on top of the cake and flip (use hot pads as the pan will be hot).Let the cake rest for 15-30 minutes. The longer it rests, the firmer the cake will become. Enjoy with a hot cuppa or coffee or dessert wine.