Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large bowl, cream the butter, brown sugar, and granulated sugar with a hand mixer until smooth and fluffy (about 2–3 minutes).
Add peanut butter and vanilla extract; mix until combined. Add the egg and mix until just incorporated.
Sprinkle in flour, baking soda, and salt. Fold together using a spatula until no dry streaks remain.
Fold in ¾ cup of the chopped Snickers pieces.
Use a 3-tablespoon cookie scoop to form 16 cookies. Place them 2 inches apart on the prepared baking sheets.
Bake one sheet at a time for 10–12 minutes, until tops are crackled and edges are lightly golden. Do not overbake.
Let cookies cool on the baking sheets for 10 minutes, then transfer to a wire rack to cool completely.
While cookies cool, make the caramel buttercream. Beat the butter until smooth and creamy.
Gradually add powdered sugar, mixing until light and fluffy.
Add 4 tablespoons of caramel sauce and vanilla extract. Beat until smooth. Adjust texture as needed: add more caramel for flavor, more sugar to thicken, or a splash of milk/cream to thin.
Once cookies are fully cooled, frost with caramel buttercream using a spoon or piping tip.
Top each cookie with the remaining chopped Snickers pieces.
Refrigerate cookies until frosting is set. Once chilled, they can be gently stacked with wax paper between layers.