Preheat the oven to 350° F.
Line a baking sheet with parchment paper.
In a large bowl, mix by hand the oil and sugar.
Mix in the eggs and vanilla extract.
Use a large spoon to mix in the flour, baking powder, and salt. Scrape the sides of the bowl to be sure to mix in all the flour to form a stiff dough.
Mix in the chocolate chunks and walnuts.
Divide the dough in half and on a floured board, shape each into a 12x4-inch log. Transfer the logs to the baking sheet.
Bake at 350° F. for 25 minutes. Put the baking sheet on a wire rack to cool for 10 minutes.
Gently transfer the logs to a cutting board. Slice the logs diagonally into one-inch slices. Wipe the chocolate off the knife after each cut. Place the slices back onto the same parchment paper-lined baking sheet.
TWICE-BAKE: Do not reduce the oven temperature for the twice-bake. Return the baking sheet with the sliced biscotti to the oven and bake the slices on one side for 5 minutes or just until you see them just slightly golden brown. Use tongs to turn them over and bake the other side for 5 minutes or until just slightly golden brown.
Immediately transfer the biscotti from the baking sheet to a wire rack to cool.
Store the biscotti covered at room temperature.