I follow my best friend Laura's recipe: it is perfect since the first time I made it, almost 10 years ago. I just added grated Tonka bean as recently I'm addicted to it and, after all, it is amazing with chocolate!
To cut a long story short, sieve flour, cocoa powder and icing sugar and pour little by little hot milk over the mixture, stirring to avoid lumps.
At this point, I added grated Tonka bean, but this filling is delicious even without it, or with vanilla.
Put the mixture on low heat and keep on stirring until it is firm, it requires just a few minutes.
Choose the sweetness you prefer adding more or less icing sugar: my filling was quite bitter, as the buckwheat cake is sweet on its own.