Preheat the oven to 350°F.
In a large bowl, beat the butter, sugar and lemon zest until light and fluffy; beat in the egg and juice. Add the flour, baking powder and salt and stir until the dough comes together.
Place on a parchment-lined baking sheet and with dampened hands (if it's sticky), shape into 12-14 inch long log, then flatten until it's 3-4 inches wide. Bake for 25-35 minutes, or until pale golden and set.
Reduce the temperature to 275°F and let the biscotti log cool. Slice on a slight diagonal into 1/2-inch thick slices and return to the baking sheet, sitting them upright, spaced at least an inch apart. Return to the oven for 30 minutes, or until crisp, pale golden and dry.
To make the drizzle, whisk together the icing sugar, cream and lemon juice. Drizzle over the cooled biscotti and leave until set.