In a medium saucepan, melt 28 g of the butter over medium heat and add the onion.
Cook until the onion is soft and translucent, then add the mushrooms and the remaining butter.
Let the mixture sweat for about 8 minutes, taking care that the onion doesn’t take on any brown color.
Stir in the stock and the a few shaving of black truffles and bring to a boil.
Immediately reduce the heat and simmer for about an hour.
Let the soup cool for a few minutes, then transfer to the blender and carefully blend at high speed until smooth.
When blended, return the mix to the pot, season with salt and pepper, and bring up to a simmer again.
Add the cognac, mix well, and serve immediately.
I like to add a few shaving of black truffle to each bowl before serving.