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.
Meanwhile, in a large bowl, whisk together the confectioner’s sugar, almond meal, white rice flour, and salt.
Add the cooled butter to the dry mixture along with the almond extract and fold in until evenly incorporated (mixture will still be very thick).
In a separate bowl, beat the egg whites until foamy. Add to the batter and fold in until fully incorporated, pressing any chunks of dough against the sides of the bowl as you go to break them up.
Divide batter evenly between prepared pans. Scatted chopped strawberries evenly over top. Sprinkle with sliced almonds and demerara sugar.
Bake financiers in preheated oven for 20-25 minutes, until tops are golden and a toothpick inserted into the center of one comes out clean. Cool briefly before transferring (in pans) to a wire rack. Allow to cool completely before removing financiers from tart pans. Financiers are best served fresh but can be kept in an airtight container at room temperature for up to a couple days.
*These can also be baking in a standard 12-cup muffin pan, but you may need to increase the baking time slightly. Look for golden tops and check for doneness with a toothpick.