The recipe we tested this weekend was wonderful and everyone loved it, including a 3 and 9 yo kids.
8 oz the bestest dark chocolate (chopped)
1/2 cup butter (I used unsalted)
1/2 tsp rum extract
4 eggs divided (I used 3 bowl method, with yolks going to the smaller bowl of my stand mixer, whites - in the big bowl, and using a small sacrificial bowl for transfers)
1/2 tsp of fine salt (no need if you using salted butter)
1/2 cup granulated sugar and 1/4 cup granulated sugar
Preheat Oven to 375 degrees F. Find either 8 or 8 and 1/2 inch spring form pan. 9 inch is too large. Spray and cover bottom with parchment paper circle. Spray the circle.
Meanwhile in a bowl large enough to contain chocolate (you won't be adding much to it) micro chopped chocolate with butter for 1 min; mix with a spatula. If not completely melted, heat for another minute. Get a chocolate goo that smells to the high heavens.
Meanwhile, beat egg yolks with 1/2 cup sugar till pale yellow and nice and smooth (I use the normal flat beater for that on my stand mixer). Mix in your rum extract. Drop some eggs into chocolate and mix in to temper, then add chocolate into eggs, incorporate. Set aside.
Get your big bowl with egg whites and the whipping blade of the mixer. Beat egg whites into soft peaks while adding your salt if you are using salt. Then beat into stiff peaks while adding the remaining 1/4 cup of sugar by sprinkling it in slowly and gently.
Get a bit of your stiff egg whites and temper your yolk-chocolate mixture with it. Then fold the chocolate-and-everything into the egg whites with the rubber spatula to achieve even consistency and not destroy the Happiness of the egg whites (that's why the measly 4 egg whites went into the biggest bowl; it was the final receptacle of everything!).
Rubber spatula the cake to be into the pan you have on the stand-by. Tap it to even the top out, put into the oven and stand back for 35 min as the chocolaty smell spreads through the house. You can spend about 30 sec taking out a wire rack. You can spend some more time whipping cream or shaving chocolate to decorate the resulting cake. It will have a dent to fill, since it is a type of a souffle.
Anyway, once 35 min is over and a toothpick comes out clean if not happy after its dive into the chocolate depths, put the spring pan on the wire rack for 15 min.
After 15 min had elapsed, ran a knife along the edge of the cake (if you oil sprayed heartily it will separate willingly), then invert the cake on the serving plate and leave like that (in the form) for another 10 min. Open the spring pan and remove it. Wait till the cake is at room temperature to peel away the parchment paper.
Dress the concave beauty any way you want and serve. :)