Preheat oven to 400ºF and butter bottom and sides of six 4 3/4″ round tart pans, making sure to cover all corners and grooves. Place pans on a baking tray.
Place butter in a small skillet set over medium-low to medium heat. Cook until the butter melts, foams up, and turns golden-brown, whisking occasionally. Remove from heat and cool briefly. Whisk in vanilla extract.
Meanwhile, in a large bowl, whisk together the almond flour, rice flour, confectioner’s sugar, cinnamon, and salt.
Add the butter to the dry mixture and fold in until evenly incorporated; note that the mixture will still be very thick.
In a separate bowl, beat egg whites until foamy. Add to batter and fold in until fully incorporated, press any chunks of dough against the side of the bowl as you fold to break them up.
Divide batter evenly between prepared pans.
In a bowl, toss peaches with melted butter, brown sugar, and cinnamon to coat. Divide peaches evenly over top of financiers.
Topping and Baking
Just before baking, sprinkle financiers with demerara sugar. Bake in preheated oven for 20-25 minutes, until tops are golden and a toothpick inserted into the center of one of the financiers comes out clean.
Allow financiers to cool completely before removing from pans. If needed, loosen edges first with an offset spatula. Store in an airtight container at room temperature for up to a couple days, or in the refrigerator for slightly longer storage. Dust with confectioner’s sugar before serving.