Preheat the oven to 350°F. Using a paper towel, generously grease 10 standard muffin cups with 2 tablespoons butter.
Melt the remaining butter in a small saucepan over medium-low heat. Add the vanilla bean seeds and pod and cook, stirring frequently, until it foams, turns clear golden, then browns and smells nutty and fragrant (be careful not to burn it!). Remove from heat and let cool while you prepare the rest (if it gets too hot, you can transfer it to another bowl to cool to keep it from cooking more). Discard the vanilla bean pod.
Whisk together the ground pepitas, oat flour, tapioca starch, sugar, and salt.
Add the lightly foamy egg whites to the dry ingredients and mix to combine.
Slowly pour the cooled brown butter into the batter and whisk just until completely combined.
Divide the batter evenly among the 10 muffin cups. Top each with two nectarine slices and sprinkle the tops with turbinado sugar to keep the fruit from drying out.
Bake for 35 minutes until golden on top and a tester comes out clean. Remove from oven and let cool for 10 minutes before removing the cakes from the pan. Optional: brush the top of the fruit with honey to make it gently glisten. Financiers are best eaten the same day, but will keep for a couple of days in the refrigerator, losing their crispy edges over time.
Notes
*You can grind up pepitas in a high-powered blender or food processor until fine (be careful not to blend too much or it might start turning to butter). Then sift the flour to remove any large chunks.*I make my own oat flour by grinding gluten-free oats in the blender until fine.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.