First, combine the mayonnaise and sour cream in a medium-sized bowl. Stir the two ingredients together until they are fully combined.
Next, add the chili powder, cumin, garlic powder, paprika, and salt to the bowl. Stir until all of the ingredients are fully incorporated and the sauce has a smooth, creamy consistency.
Taste the sauce and adjust the seasonings to your liking. If you prefer a spicier sauce, you can add more chili powder or even a pinch of cayenne pepper. If you prefer a milder sauce, you can reduce the amount of chili powder or omit it entirely.
Once you're happy with the flavor of your Baja sauce, cover the bowl and place it in the refrigerator to chill for at least 30 minutes. This will allow the flavors to meld and intensify.