1. Beat the eggs with sugar. Add vanilla, salt, butter, and zest.

2. In a large bowl, sift flour and baking powder.

3. Add the egg-sugar mixture to the flour and mix well. Add soluble coffee and crushed coffee.

4. Divide the dough into 2 pieces. Wrap each piece in plastic wrap. Refrigerate for 1 hour.

5. Preheat oven to 275°F.

6. On a lightly floured surface, knead each ball of dough for 1 minute. Roll out the dough into an oval shape with your hands.

7. Place on a buttered cookie sheet and bake. Cook for about 15 to 20 minutes.

8. Remove from oven and let cool on a rack. Once cooled, with a bread knife, cut each cake into equal slices. Place these slices on a baking sheet and bake in the oven at 150°F for about 15 minutes.

9. Remove from oven; place biscottis on a rack and let cool completely.