In a small saucepan, cook the butter over medium heat stirring frequently until dark and golden but not burnt.
Meanwhile, combine the sugar, brown sugar, almond flour, all-purpose flour, baking powder, salt, and lemon zest in a medium bowl. Once butter is browned, add it all at once to the dry mixture and quickly beat to incorporate. Set the batter aside for about five minutes to allow it to cool slightly then mix in the vanilla and egg whites. Cover the bowl with plastic wrap and pop it in the fridge to chill for at least one hour or up to 24 hours.
When ready to bake, preheat the oven to 375F and lightly grease a muffin tin or financier pan with cooking spray. Sprinkle each lightly with a little more almond flour and shake to coat. Divide the batter into the prepared pan and bake for 18–22 minutes for a muffin tin or 20–25 minutes for a silicone financier mold or until golden and springy.
Let the financiers to sit in the pan for about five minutes, then turn them out and allow them cool on a wire rack before serving.