Step 1Heat the oven to 350 degrees. Line a baking sheet with parchment paper.
Step 2In the bowl of a stand mixer, or in a large bowl using a hand mixer, beat together the butter, sugar, salt, vanilla and baking powder until smooth and creamy. Beat in the eggs, one at a time (the batter may look slightly curdled). Over low speed, beat in the flour, mixing until smooth. The dough will be soft and sticky.
Step 3If you would like to add finely chopped nuts, chocolate chips, seeds or dried fruit, you can do it after mixing in the flour. (For example, I added one-fourth cup of poppy seeds and 1 cup of chopped pecans.)
Step 4Shape the dough into a rough log about 14 inches long, 2 1/2 inches wide and about three-fourths of an inch thick. Place the log on the cookie sheet and bake for 25 minutes.
Step 5Remove from the oven and lower the temperature to 325 degrees. Cool the log on the cookie sheet for 20 minutes.
Step 6Cut the biscotti into straight one-half inch slices. Arrange them so they stand upright on the cookie sheet, keeping a little space between the slices.
Step 7Bake for another 20 minutes.
Step 8Cool completely before storing in an airtight container or in the freezer.