Add oil to skillet with the jalapenos and cook on medium heat, turning occasionally, so all sides of the jalapenos get charred and blistered, approximately 6-10 minutes.
Add the garlic to the skillet and cook for about 30 seconds, turn over the garlic and cook another 30 seconds. If it's too brown, it will add a sweetness to the aioli.
Put the mayo in a food processor or blender. Transfer the jalapenos, garlic, and any leftover oil in the skillet to the blender. Add the onion powder and black pepper and blend until smooth. (You may need to scrape the sides of the blender). Enjoy! (see notes below)
Video
Notes
I don't add salt because mayo already contains salt. Feel free to taste the aioli while it's still in the food processor and add some if desired.
To make it less spicy, after the jalapenos have roasted, cut them open and scrape out the seeds and membranes.
Store aioli in an air-tight container in the fridge and use within 5 days. (I like to put tape on my container with the date I made it)
Always use a clean utensil to prevent contamination.