- PRHEAT oven to 325F. Line a large baking sheet with parchment. Stir flour with baking powder and salt in a medium bowl.
- BEAT butter with sugar in a large bowl, using an electric mixer on medium-high, until pale and fluffy. Beat in eggs, 1 at a time, then lemon zest and lemon juice. Stir in flour mixture until combined. Gather dough together and divide in half on prepared sheet. Shape each portion into a 2-in.-wide and 3/4-in.-high log. Place logs 3 in. apart, as they will spread.
- BAKE in centre of oven until tops are almost firm but still golden, 30 to 35 min. Remove from oven and let stand until loaves are cool enough to touch, about 10 min. Cut each diagonally into 1/2-in. slices. Place a rack on baking sheet. Arrange sliced biscotti on rack, cut-side down. Continue baking until very firm and golden brown, 20 to 30 more min.
- BEAT egg white with lemon juice. Beat in icing sugar until smooth. Brush meringue icing over tops of cooled biscotti. Caramelize tops with a kitchen torch.
- STORE in an airtight container up to 1 week or freeze up to 1 month.
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