Preheat oven to 325, and grease a large parchment-lined baking sheet.
Sift flour, baking powder, espresso powder and salt into a medium bowl. Add toasted hazelnuts, which I like to break into smaller pieces.
In another bowl, combine orange zest, sugar, oil, eggs and vanilla extract. Whisk until smooth.
Add egg mixture to flour mixture. Stir with wooden spoon and blend well.
Divide dough in half and shape each into a cylinder (roughly 12 x 2-inches). Arrange cylinders on the baking sheet, leaving at least 3-inches between them.
Bake for 30 to 35 minutes or until golden and firm. Remove from oven and let stand for 15 minutes before cutting.
Transfer cylinders to a cutting board and carefully slice crosswise on the diagonal, with a serrated knife, into ½-inch slices. Put slices back onto the baking sheet, laying them flat and not overlapping.
Bake a second time for 20 minutes, turning them over after 10 minutes.