Preheat the oven to 350 degrees F.
Line a heavy baking sheet with parchment paper. Sift together flour and baking powder and set aside. In a mixer, beat the sugar, butter, lemon zest and salt. Beat in the eggs, 1 at a time. Add the flour mixture and beat just until blended. Stir in the cranberries and 1/2 of the white chocolate chips.
Form the dough into a 12-inch long, 3-inch wide log on the parchment lined baking sheet. Bake for 40 minutes. Remove and let cool completely.
Place the log on the cutting board. Using a sharp serrated knife, cut the log on a diagonal into 3/4-inch-thick slices. Arrange the biscotti on the baking sheet. Bake the biscotti again, for 15 minutes, until they are golden brown. Transfer the biscotti to a rack and cool completely.
Melt the remaining white chocolate chips in a double boiler. Dip the bottom of the biscotti in the white chocolate and let chocolate cool and set.