Stir shortening and butter flavoring together in a bowl until well combined.
Stir sugar, peanuts, corn syrup, water, and salt together in a 2-quart saucepan. Heat mixture over medium-high heat, stirring frequently, until temperature reaches 300 degrees F (149 degrees C), or until a small amount of syrup dropped into cold water forms hard, brittle threads, about 15 minutes. Remove from heat.
Carefully stir baking soda and vanilla into peanut mixture until fully incorporated. Stir shortening mixture into peanut mixture until dissolved.
Pour peanut mixture in a single layer onto prepared baking sheet; cool until hard. Crack brittle into pieces.
If using popcorn salt you may want to leave out the regular salt. I found it was not too salty for me, but it is per your personal taste.