Following is for a 240 mm round baking tray. You need a springform tray (one where the rim can be removed) or one of those where the bottom comes out.
100 g dark chocolate (go for quality and high cocoa content)
150 g soft unsalted butter
150 g sugar
4 free range medium size eggs
50 g ground almonds
50 g corn starch
50 g normal white wheat baking flour
50 g fine dried bread crumbs
1 tablespoon of baking powder (unless you use self-rising flour)
1 tablespoon of vanilla sugar (or equivalent amount of vanilla extract, aroma or genuine vanilla)
Melt the chocolate in a bowl over hot water or *very* carefully in the microwave oven at very low power setting. This works quicker but it is easy to burn the chocolate in the micro.
Mix butter, sugar and vanilla, whisk until slightly foamy.
Separate the egg yolks from the whites, whisk the egg whites stiff.
Mix almonds, flour, corn starch, bread crumbs. Add a tablespoon of dark cocoa for an extra boost if you want.
Check that the chocolate has cooled down but is still runny or very soft. Must be under 80 C so that the eggs won’t cook in it.
Mix the egg yolks into the chocolate.
Add the almonds, etc, to the eggs and chocolate. Mix.
Gently add the egg whites to the mix. Traditional cooking wisdom calls this to “fold in the whites” using a thin metal spoon, careful not to knock out too much air. I prefer using a whisk, just don’t use it for whisking. Instead gently rotate it to fold the egg whites into the batter mix. I find it is easier to evenly distribute the whites using the whisk; the traditional spoon method is likely to break more of your stiff whites.
Preheat the oven to 180 C. Use Top and bottom heat or convected air.
Butter the form really well, especially around the corners.
Bake for 45 minutes.
Rest for 3 hours. Remove it from the tin when the time is right. Too soon and it will break, too late and it will be soggy. You need to judge and find the sweet spot, but if you have a good baking tray where the sides come off or the bottom can be pushed up, the highest risk is in that you burn your hand, wrist or arm on the hot tin. Every time…
Slice once or twice, douse with booze.
Make the filling, assemble and decorate.
Afterthought, June 2020: Mother’s Express Sponge makes a fast, easier and ligher base for this.