Preheat the oven to 160℃/Fan℃140/Gas 4. Grease and line a 20cm loose-bottomed cake tin.
Melt the chocolate and butter in a heatproof bowl over a half-filled pan of boiling water, making sure the bowl doesn’t touch the water.
Alternatively, you can do this directly over a very low heat, just be sure to stir it all the time to prevent the chocolate from burning. Set aside to cool.
In the bowl of a free-standing mixer, whisk the eggs and sugars until thick and pale. Mix the rye and cocoa together in a small bowl. With the mixer on a low speed, carefully add the cocoa mixture to the eggs, a spoonful at a time.
Next, whisk your chosen booze into the cooled chocolate, then pour this mixture in a slow stream into the egg mixture.
Turn off the mixer, then stir gently by hand once fully combined to make sure no chocolate has sunk to the bottom. Pour into the prepared tin from a low height so as not to lose any aeration.
Bake for 20–25 minutes, until the cake has nearly set but still has a wobble in the middle.
Allow to cool completely in the tin. Dust with a little cocoa, then serve with a big dollop of crème fraîche.