Step 1
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.
Step 2
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.
Step 3
Preheat the oven to 350 degrees F (175 degrees C). Lightly grease two cookie sheets.
Step 4
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.
Step 5
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.
Step 6
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.
Step 7
Bake for 8 minutes then turn the cookies over and bake for an additional 8 minutes or until lightly toasted on each side. Cool completely then store in an airtight container.