The brachot site mentions three important rules in defining ikar / tafel that seem to be relevant to the PayDay bar:
I. Tofel by Function (Serving Tofel or Tofeil HaMeshameshet)
Definition: is when you have two separate food items and one item
serves the other item. The Tofel is not eaten for its own enjoyment,
but only for the sake of the Ikar, in that sense we say the tofel
serves the Ikar. Simply put: had you not eaten the Ikar, then you
wouldn't eat the Tofel either.
Most people eat a Payday because of the peanuts. The peanuts are also on the outside and the caramel is a "binder".
An alternate possibility applicable to Payday is:
III. Tofel by Form (Mixtures)
Definition: If the ingredients are cooked or baked together and both
ingredients are typically eaten on one spoonful it is considered a
single entity. Also if the ingredients are mixed or blended so that
one spoonful contains both foods it is considered a single entity. In
such cases, only one bracha is made and it is upon the ikar component
within the mixture.
I doubt that the peanuts and caramel are baked together. But, they are eaten together, typically. (If one intentionally ate the peanuts first and then the caramel and intended to enjoy both, there are some opinions stating that he should make separate brachot.)
Lastly, even if one were to question if the caramel enhances the peanuts or the vice versa, we would follow this rule:
If neither ingredient enhances the other the majority item is ikar
Ingredients are listed in reverse quantity order. This site confirms that peanuts are the majority ingredient.
based on all the above, I would infer that the bracha should be Ho'adamah.