Prepare the Financier Batter

1. Brown the Butter: Start by melting the butter in a small saucepan over medium heat. Once melted, continue to cook for about 5 minutes, swirling occasionally, until the butter turns golden brown and develops a nutty aroma. Set it aside to cool slightly.
2. Preheat the Oven: Preheat your oven to 375°F (190°C). Grease your financier molds or line a muffin tin with paper liners.
3. Mix Dry Ingredients: In a large bowl, whisk together the almond flour, all-purpose flour, powdered sugar, and salt.
4. Whisk the Egg Whites: In a separate bowl, lightly whisk the egg whites until frothy but not stiff.
5. Combine the Wet and Dry Ingredients: Add the whisked egg whites, vanilla extract, and almond extract (if using) to the dry ingredients. Stir until the mixture is smooth.
6. Add the Browned Butter: Pour the browned butter into the batter and mix until fully incorporated. The batter should be thick but pourable.

Bake the Financiers

1. Fill the Molds: Spoon the batter into the prepared molds, filling each one about two-thirds full.
2. Bake: Place the molds in the preheated oven and bake for 15-20 minutes, or until the edges are golden brown, and a toothpick inserted into the center comes out clean.
3. Cool: Let the financiers cool in the molds for 5 minutes before transferring them to a wire rack to cool completely.