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Preheat oven to 375.
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In a mixer, beat butter for about 30 seconds to soften. Add 1 C of flour, the sugar, egg yolks and vanilla.
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When thoroughly combined, beat in remaining 1/2 c flour.
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Chill dough for 1 hr.
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Lightly beat egg whites and place in a shallow bowl or tray. In another shallow bowl or tray place walnuts.
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Shape dough into 1 inch balls and roll each in the egg white then the nuts. Place on a cookie sheet 1 inch apart (they will not spread much at all).
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Use your thumb to press in the center of each cookie to make a "well" for the jam eventually to go into.
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Bake cookies for 10-12 minutes, until edges are lightly browned.
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Cool fully. Just before serving place a dollop of jam, jelly or fruit butter into the center of each cookie.