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Preheat the oven to 350°F. Lightly grease (or line with parchment) one large (about 18" x 13") baking sheet.
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In a medium-sized bowl, beat the butter, sugar, salt, zest, almond extract, and baking powder until the mixture is smooth and creamy. Beat in the lemon juice and eggs; the batter may look slightly curdled. At low speed of your mixer, add the flour, stirring until smooth; the dough will be sticky.
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Scrape the dough onto the prepared baking sheet. Shape it into a log that's about 13" long x 3" wide x 3/4" thick. Straighten the log, and smooth its top and sides; a wet spatula or wet bowl scraper works well here.
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Bake the dough for 25 minutes. Remove it from the oven, and allow it to cool on the pan anywhere from 10 to 25 minutes; just work it into the schedule of whatever else you're doing in the kitchen. Reduce the oven heat to 325°F.
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Using a spray bottle filled with room-temperature water, lightly but thoroughly spritz the log, making sure to cover the sides as well as the top. Softening the crust just this little bit will make slicing the biscotti much easier.
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Wait another 5 minutes, then use a serrated knife to cut the log crosswise into 1/2" slices. As you're slicing, be sure to cut straight up and down, perpendicular to the pan; if you cut unevenly, biscotti may be thicker at the top than the bottom, and they'll topple over during their second bake.
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Set the biscotti on edge on the prepared baking sheet. Return the biscotti to the oven, and bake them for 30 to 35 minutes, until they feel very dry and are beginning to turn golden around the edges. Remove the biscotti from the oven, and transfer them to a rack to cool.
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To make glazed biscotti: Combine the confectioners' or glazing sugar with the lemon juice powder and milk; drizzle over cooled biscotti.
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Store completely cool biscotti at room temperature, well wrapped, for a couple of weeks. Freeze for longer storage.
