Put the butter in a small saucepan and cook over medium heat until it melts and only just begins to take on a pale golden color. Pour the butter into a measuring cup with a spout and set aside.
Use a fork to stir the egg whites in a small bowl just enough to break them up; set aside.
Put the pistachios and the sugar in a food processor and pulse until the nuts are ground (it's okay if you have some chunks here and there). Add the flour and salt; pulse again to blend, and then transfer the mixture to a medium bowl.
Pour the egg whites into the bowl and using a whisk or spatula, stir gently until they are blended into the nut mixture. Gradually add the butter to the mixture. It will seem like a LOT of butter, but keep stirring lightly and soon you'll have a thick, shiny batter. Perfection!
Press a piece of plastic wrap against the surface of the batter and chill for at least 8 hours (up to 2 days).
Center a rack in the over and preheat the oven to 350 degrees F. Spray the molds of a mini-muffin tin with nonstick spray and place the pan on a baking sheet.
Fill each mold half full with the chilled batter. Place 1 raspberry in the center of each cup.
Bake the financiers for 24 to 28 minutes, rotating halfway through. The tops will be springy to the touch and a toothpick inserted in the center will come out clean when the cookies are done. Run a table knife around the edges of the muffins to detach them from the pan then cookies onto a rack. Allow to cool to room temperature.
Recipe by Kitchen Meets Girl at https://kitchenmeetsgirl.com/pistachio-and-raspberry-financiers/