• Make the brown butter: Heat the butter in a small stainless steel saucepan (see notes) over medium heat until completely melted and simmering. Keep on cooking over low heat, swirling the pot from time to time. The butter will bubble up as the water evaporates, which will prevent you from watching closely over the color changing. When that happens, lift the pot off the heat for a few seconds, swirling it gently until the bubbles recede, then put back on the heat. The butter is ready when the milk solids at the bottom of the pot turn a light brown color and the concoction gives off a delicious hazelnut aroma.
• When the butter is browned, remove the pan from the heat, pour in a small bowl, and let cool completely for 10 minutes.
• In a large mixing bowl, whisk together the almond flour, all-purpose flour, granulated sugar, brown sugar, baking powder, and citrus zest, if using. Add the egg whites and whisk until they are fully incorporated and the mixture is thick and somewhat sticky. Mix in the brown butter and vanilla, if using, whisking gently at first to incorporate, then giving it a vigorous stir to allow the batter to fully come together. Transfer the batter to an airtight container and refrigerate for 1 hour, or for up to 1 week.
• When ready to bake the financiers, preheat the oven to 350°F (175°C). Line a muffin pan or a mini muffin pan with parchment paper liners (see notes) or grease a financier pan with softened butter, then sprinkle with flour and tap out the excess. (Make sure to grease the pan even if it is made of silicone.)
• Drop a tablespoonful of dough in each cup. (You can use a small cookie scoop to do this quickly.) Sprinkle each financier with fruits or toppings, if using. Bake for about 12 minutes for mini muffin pans, or about 15 minutes, for regular muffin pans, rotating the pan halfway through. The financiers are done when they are golden brown around the edges and set in the center. Let cool for 10 minutes in the pan, then transfer to a cooling rack to cool completely.
• STORAGE: Store in an airtight container at room temperature for up to two days. Always bring the financiers back to room temperature before eating for the best flavor.
• French financiers can also be frozen. Store them in heavy-duty freezer bags for a maximum of 1 month. Thaw the financiers in the fridge overnight, or at room temperature for 30 minutes.
• To return thawed French financiers to their freshly baked texture, reheat them in a 325°F (160°C) oven for 6 to 8 minutes, then let them cool to room temperature before serving.

NOTES:

• This recipe recommends using regular muffin pans or mini-muffin pans because they can commonly be found in most kitchens. You can also use a specialty financier pan, but keep a close eye during the baking process and adjust the baking time accordingly.
• To brown butter, it’s best to use a stainless steel saucepan because it’s easier to keep a close eye on the butter changing color against the pale gray metal. If you use a non-stick pan, you can use a spoon to scoop up some butter to check for doneness throughout the cooking process (just make sure to dip the spoon all the way down to the bottom of the pan to get some of the milk solids).