Place the raspberries and 2 tablespoons of sugar into a small saucepan over medium heat. Bring to a boil. Cook, stirring occasionally, until reduced by about 1/3. Remove from heat, and strain through a sieve to remove the seeds. Set aside to cool.

In a large bowl, cream together the butter and 1 1/3 cups of sugar until light and fluffy. Beat in the eggs one at a time, mixing well after each one. Stir in the raspberry sauce and lemon zest until well blended. Combine the flour, baking powder, and salt; stir into the batter. Cover and refrigerate for about 10 minutes.

Preheat the oven to 350 degrees F (175 degrees C). Lightly grease two cookie sheets.

Divide the dough into four equal parts. Form each piece of dough into a 12-inch long log shape on a lightly floured surface. Place two logs onto each cookie sheet, and pat down until they are about 3 inches wide. There should be at least 2 inches of space between each log.

Bake for 30 minutes in the preheated oven, until firm but not crisp. Remove from the oven, and allow to cool for at least 10 minutes.

Place the biscotti bars on a cutting board, and use a serrated knife to cut into 3/4 inch wide slices. Return the slices to the baking sheets cut side down.