Preheat the oven at 220° C (428° F).
Melt the butter.
In the bowl of the stand mixer fitted with paddle attachment (or in a large bowl using a whisk) combine almond flour, confectioner sugar, cocoa powder and salt.
Add the egg whites. When they are incorporated add the buttermilk.
Drizzle in the melted butter while the mixer is going.
Sift together the flour and baking soda, and add them to the mixture.
Transfer the batter in a pastry bag without tip.
Fill each cup of a mini muffin mold to ¾ of its capacity (they will raise during baking).
Bake at 220° C (428° F) for 5 minutes. Then lower the temperature to 200° C (392° F) and bake for another 6 minutes.
Take out of the oven and let them cool off 10 minutes before placing each finacier on a cooling rack.
Sprinkle some confectioner sugar before serving.