Place the vanilla beans in one of two small bowls, and the raisins in another; pour half the sherry over each. Let sit for 25-30 minutes.
In a large skillet, saute the chicken breasts in the melted butter for 5 minutes on each side, over medium heat.
Remove chicken from pan, add onions, and saute until the onions are transparent. Add flour and stir continuously with a wire whisk until the flour has taken on a golden color (the classic French "roux.".)
Add chicken broth all at once, stirring continuously with the wire whisk, until the broth has thickened.
Lower heat, add chicken breast pieces, vanilla beans and raisins with sherry liquid and continue cooking over medium-low heat an additional 15 minutes.
The classic accompaniment to this dish in the Sierra Huasteca is a sweet corn tamale, which is dipped into the sauce. Alternatively, it may be served with tallarines, wide noodles.
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