Make sure your tofu is really well pressed. You want these bites to be firm so they'll stay on the toothpicks. Slice the tofu in half lengthwise through the center, giving you two large flat rectangles. Stack on top of each other, then slice into thirds. Slice again into thirds in the other direction. This will give you 18 bite-sized cubes. If you want smaller bites, slice into fourths.
Mix the soy sauce, sesame oil, black pepper, garlic powder and lemon juice, then add the tofu. Let them hang out together in the fridge for about an hour. Preheat the oven to 350 degrees. Cover a baking sheet with aluminum foil and spray with a little cooking spray. Spread the tofu out on the foil and bake for 20 minutes. Remove, spray the tops of the tofu with more non-stick spray, then flip each piece.
Cover with the remaining soy sauce mix, and add a few cracks of pepper, then back for another 15-20 minutes or until they're firm and are starting to get dark and crispy around the edges. While the tofu is in the oven, mix together the mayonnaise and the chili powder for your dipping sauce. Chill until ready to serve. When the tofu is done, serve 1-2 per toothpick.