Preheat oven to 220 C or 425F. Line a baking tray with baking paper.
To make the spice mix, combine together garlic powder, ground cumin, paprika and cayenne pepper (if using). Season with salt and pepper. Set aside.
Using a sharp knife, cut off the bottom (about 1 cm) of the corn cobs. Peel and discard the husks. Remove the fibers underneath.
Cut the corn cobs into halves. Sit the corn in the flat base, and cut into three equal slices. Make sure the middle is intact to hold the rib together.
Coat the ribs in oil and spice mix.
Arrange them on the baking tray in a single layer.
Bake for 20-30 minutes or until curled and crispy.
Serve immediately.