Note: Imported varieties of coarsely ground cornmeal may be labeled "polenta"-that's what you need for this recipe. Porridge made from the cornmeal is also called polenta, so it can be confusing.
Position racks in the upper and lower thirds of oven; preheat to 325ºF. Coat 2 large baking sheets with cooking spray.
Whisk flour, sugar, cornmeal (or polenta), baking powder and salt in a large bowl. Whisk eggs, oil and vanilla in another bowl until frothy and well combined. Add the egg mixture to the flour mixture and stir with a spoon until a soft dough forms.
Turn the dough out onto a well-floured work surface. Divide it in half and shape each half into a log 12 inches long by 2 inches wide. Brush off excess flour and place the logs on one baking sheet.e the logs on the upper rack until almost firm when pressed on top, 20 to 25 minutes. Remove from the oven and let cool on the pan for 20 minutes; reduce the oven temperature to 300º.
Place the logs on a cutting board and slice diagonally into 1/2-inch-thick slices using a serrated knife. Divide the biscotti between the 2 baking sheets, standing them up about 1 inch apart.
Return the biscotti to the oven and bake until lightly colored and dry, 20 to 25 minutes. Transfer to wire racks to cool (the biscotti will crisp as they cool).