Line a large cookie sheet with parchment paper or foil coated with baking spray.
In a large bowl, stir the butter with the sugar.
Beat in the eggs, one at a time, then the Marsala, zest, lemon juice, vanilla and salt until just combined.
In another bowl, stir the flour with the baking powder.
Stir the flour mixture along with the chopped almonds into the egg mixture until combined; the dough will be sticky.
Divide the dough in half; dollop lengthwise on the lined baking sheet.
Wetting your hands with cold water, flatten the dough into long rectangular logs about 1/2 inch thick extending from one end of the sheet to the other. Logs should be at least 2 inches apart on the sheet.
Sprinkle the tops with slivered almonds; gently pat down the dough.
Bake in the centre of the oven until lightly golden on the edges and firm to the touch, about 30 minutes.
Slide the parchment or foil onto a cutting board.
Using a large sharp knife, cut crosswise into the slices about 1/2 inch thick.
Picking up the segments, stand the biscotti upright about 1/2 inch apart on the unlined cookie sheet.
Reduce the oven temperature to 300 degrees F; bake the biscotti in the centre of the oven for 35 to 40 minutes or until lightly golden and crisp.
Cool on a rack, then store in an airtight container for up to a month, or freeze.