The Bojon Gourmet

1 Position a rack in the center of the oven and preheat to 350ºLine a standard muffin pan with paper liners (alternatively, grease the pans with softened butter and dust with oat flour, tapping out the extra.)
2 Melt the butter with the vanilla pod and scrapings in a medium saucepan over a medium flame. Continue cooking until the butter turns golden and smells nutty, 5-10 minutes, swirling occasionally.
3 Meanwhile, place the poppy seeds in a heatproof measuring cup. When the butter has browned, pour it into the cup with the poppy seeds. This will stop the cooking and enhance the flavor of the seeds. Set aside to cool at least 15 minutes. Remove the vanilla pod (you can rinse it, let it dry, and use it to make vanilla extract).
4 Meanwhile, in the bowl of a food processor combine the almonds, oat flour, rice flour, powdered sugar, muscobado sugar, and salt. Process until the almonds are finely ground. Blend in the egg whites until combined. With the motor running, pour in the cooled brown butter, leaving the poppy seeds in the bottom. (This helps the butter emulsify into the batter.) Turn off the motor, and stir in the poppy seeds and any remaining butter with a flexible spatula. (It helps to remove the blade.) The batter will be quite fluid. Divide the batter among the lined muffin cups; it will come about halfway up the sides.
5 Halve the plums and remove the pits. Place a half cut-side down, and cut with a sharp, serrated knife into 1/4" thick slices, discarding the two end pieces. Repeat with the other plums. Fan four or so slices atop each financier, and sprinkle the tops with coarse sugar.
6 Bake the financiers until they are puffed, golden on top, and a toothpick inserted into the center comes out clean, 25-35 minutes. Remove from the oven and let cool until warm; the cakes are still cooking from residual heat.
7 Serve the financiers warm or at room temperature.