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.