Place a rack in center of the oven and preheat the oven to 350 degrees F. Line
a baking sheet with parchment paper and set aside.
Place the cake mix, melted butter, eggs, flour, and macadamia nuts in a large
mixing bowl. Blend with an electric mixer on low speed until well blended, 3 to
4 minutes. Stop machine and scrape down the sides of the bowl with a rubber spatula.
The dough should come together into a ball. Transfer it to the prepared baking sheet.
With floured hands, shape the dough into a rectangle about 14 inches long by 4 inches
wide by 1/2 inch thick. Mound the dough so it is slightly higher in the center.
Place the baking sheet in the oven.
Bake the biscotti rectangle until it feels firm when lightly pressed with your
finger and a wooden pick inserted in the center comes out clean, 30 to 35 minutes.
Remove the baking sheet from the oven and let the biscotti cool 10 minutes. Leave
the oven on.
Cutting on the baking sheet, use a sharp serrated bread knife to slice the rectangle
on the diagonal into 1 inch thick slices. You should get 14 slices. Carefully turn
these slices onto their sides, using the slicing knife to arrange them on the same
baking sheet. Return the baking sheet to the oven. Bake the biscotti 10 minutes.
Turn the oven off and let the biscotti remain in the oven until they are crisp,
for 30 to 40 minutes more.
Remove the baking sheet from the oven, transfer the biscotti to a rack and allow
them to cool completely, 2 hours.