Black Sesame Kumquat Financiers {Gluten-Free}

Let's make financiers!:
2 Position a rack in the center of the oven and preheat to 350ºF. With a pastry brush dipped in the soft butter (it should be the consistency of mayonnaise; melted butter may not work adequately, especially if your muffin pans are nonstick), generously grease 12 standard muffin cups or 24 mini-muffin cups, brushing butter over the top of the pan as well to discourage sticking.
3 Place the remaining 1 1/2 sticks of butter in a medium, heavy-bottomed saucepan with the vanilla pod and scrapings. Cook over a medium flame until the butter foams up, turns golden, and smells nutty, 5-10 minutes. Let cool while you prepare the rest, 10-20 minutes. Remove the vanilla pod and discard.
4 Meanwhile, in a small, dry skillet, toast the sesame seeds over a medium-low flame, shuffling the pan regularly, until the seeds begin to pop and smell fragrant, 2-3 minutes. Remove the seeds to a plate to cool completely, then grind finely in a clean coffee or spice grinder.
5 In a large bowl, whisk together the toasted and ground black sesame seeds, almond flour, rice flour, oat flour, sugar, and salt. Whisk well to eradicate lumps. Whisk in the egg whites vigorously until the batter is smooth, then whisk in the melted, cooled butter little by little until well-combined.
6 Using a sharp knife, cut the ends off of the kumquats, reserving the ends, and slice the kumquats thinly into 4-6 slices each. Use the tip of a paring knife to remove any seeds. You'll want 4-5 slices for large cakes or 3 slices for minis. When you have enough slices for the tops, cut the remaining kumquats and the reserved ends into small bits. You should have about half a cup. Stir the kumquat pieces into the batter.
7 Divide the batter among the greased cups, filling them two-thirds to three-quarters of the way to the top. Top with a fan of kumquat slices and sprinkle the kumquats generously with sugar (this will keep them from drying out as they bake) and a few sesame seeds. Bake the cakes until golden on top and a tester inserted in the center comes out clean, 35-45 minutes for large cakes, 25-35 minutes for small cakes. Remove from the oven and let cool 10 minutes, then use a small butter knife or offset spatula to loosen the edges and release the cakes from the pans.
8 I like these financiers best the day of baking, when still a bit warm from the oven. The edges are crisp and the middles are soft and pillowy, with bits of moist citrus throughout. Extras keep well, airtight at room temperature and layered on parchment paper, for an additional day or two.