Prepare the brown butter: in a small saucepan, heat the butter in pieces over very low heat, stirring, until it takes on an amber-caramel hue. Immediately remove from heat, sieve to remove the foam and pour into a bowl to stop the cooking. Let cool for the rest of the recipe.
Preheat the oven to 200° traditional heat. Butter the financier mold (except if the silicone one).
In a large container, sift the ground almonds, flour, icing sugar and salt. Mix everything for a fine and homogeneous texture.
Separately, loosen the egg whites with a fork (without whisking them). Then add them to the dry ingredients with the vanilla. Mix with a spatula/maryse. There should be no lumps. Do not overmix, just as much as needed.
Finish by adding the warm brown butter, incorporate by mixing gently with the spatula/maryse.
Pour the batter into the financial mold using a piping bag (for more precision) or using a tablespoon. Leave 2 millimeters of empty space in each indentation to prevent it from overflowing during cooking. Optional: sprinkle the top with a few slivered almonds.
Bake for about 11-13 minutes (adjust according to your oven, as they are all different). The angles and the top of the financiers must be gilded.
Leave to cool in the mold then unmold on a wire rack until completely cooled.
Notes
Financiers will keep at room temperature, in an airtight container, for about 4-5 days. I do not recommend the refrigerator because they would harden.