Heat butter in a saucepan on medium heat, first it will melt then it will foam. It is ready when the white bits on the bottom of the pan turn into golden brown and it has a nutty smell. Make sure you carefully watch it throughout the process, stir regularly and remove from the heat before it starts burning. Let it cool to room temperature
In a bowl whisk egg whisk for a few minutes until foamy, then whisk in powdered sugar
Whisk in brown butter, then divide the batter into 3 if making all 3 flavors
One should be folded with flour, hazelnut flour, salt. The other should be folded with flour, almond flour, salt. The third part should be folded with pistachio flour, flour, salt. You can of course chose to make only 1 flavor only, in that case no need to divide the batter
Move each cake batter into a piping bag and let them rest in the fridge for an hour
In the meantime, pre-heat your oven and prepare your financier mold
Ideally using a piping bag fill the tin with the 3 different cake batter, then, before baking, hint them with chopped roasted nuts (pistachio, hazelnut, almond)
Bake at 175C / 347F for 20 minutes
Once they are baked, let them rest for 2-3 minutes in the tin then remove them.
They are the best eaten fresh. Any leftover can be stored in an airtight container at room temp for a few days.