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1
Preheat oven to 400 F. Generously butter 12 financier tins or a 12-cup muffin pan.
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2
In a dry frying pan, toast the ground walnuts until fragrant and lightly colored (you can also toast the walnuts before grinding, if you prefer). Do not allow to burn. Set aside in a small bowl and wipe out the frying pan.
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3
Cut the butter into five large pieces and add to the frying pan. Melt over medium heat. When the butter starts to boil, add the sage leaves. Keep a close watch on the pan and remove from the heat when the butter is fragrant and nutty-smelling, with small brown flecks, and the sage leaves are crisp. Pour into a medium bowl. For a perfectly smooth texture, strain the butter; otherwise, just fish out the sage leaves to use later as a garnish.
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4
Add the walnuts, flour, maple syrup, and powdered sugar to the butter and whisk thoroughly. Add the egg whites and whisk some more, until the mixture is completely smooth. It should be pourable. Pour the batter into the prepared tins and press a sage leaf onto the top of each.
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5
Bake for 12-15 minutes, or until the tops are bouncy (if you're using a muffin tin, it might take a few minutes longer). Turn off the oven and leave the financiers in to dry for 5 more minutes. Remove from oven and allow to cool for five minutes, then unmold and serve.
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6
Financiers will keep, in an airtight container at room temperature, for up to three days. I think they're even better the next day, when the texture is a bit denser.
