1. Preheat the oven to 375ºF/180ºC and butter the insides of 24 mini muffin tins generously with softened, not melted, butter, making sure the butter the upper rims of the indentations.
2. In a medium bowl, mix the almond or hazelnut powder, sugar, flour and salt. Stir in the egg whites and vanilla or almond extract, then the browned butter.
3. Fill each indentation of the mini muffin tins almost to the top. Rap the tins sharply on the counter to level the tops, then bake for 13 minutes, until nicely browned. Let the financiers cool in the tins, then remove them, using a sharp knife to help release them, if necessary.