5 ounces (1 ¼ sticks) unsalted butter, diced, plus more for greasing the tins
5 ½ ounces sugar
2 ½ ounces all-purpose flour, plus more for dusting the tins
2 ½ ounces almond meal
5 ounces egg whites, at room temperature
3 Italian prune plums, halved, pitted, and thinly sliced
Heat a small, heavy saucepan over medium heat until very hot but not smoking. Add the butter and cook, stirring constantly, for 5 to 6 minutes, or until browned. Transfer the butter to a bowl and let cool.
Whisk together the sugar, flour, and almond meal in a large bowl. Whisk in the egg whites until thoroughly combined and then whisk in the butter until thoroughly combined. Cover with plastic wrap and let rest for 1 to 2 hours.
Preheat the oven to 375˚F. Butter and flour 12 financier tins and divide the batter among them. Fan out the plum slices atop the batter in each financier tin. Bake for 20 to 22 minutes, or until golden brown and the edges of the financiers start to shrink away from the tins. Let the financiers cool in the tins for about 10 minutes. Invert onto cooling racks and finish cooling completely.
Makes 1 dozen financiers. If you do not have traditional rectangular financier tins, use a muffin pan and adjust the baking time accordingly. The brown butter should be cooled but still melted when you mix the financier batter.