Preheat oven to 350°F and line two baking sheets with parchment paper.
In a large bowl, use a hand mixer with beaters to cream together softened butter and the sugars. Beat until totally combined and smooth, about 2-3 minutes.
Add peanut butter and vanilla extract and beat until combined. Add egg and beat until just combined.
Sprinkle the flour, baking soda, and salt over the mixture and use a spatula to fold everything together until combined and no streaks of flour remain. The dough will be soft and a bit sticky.
Use a spatula to gently fold in 3/4 cup of the chopped Snickers candy bar pieces.
Use a 3 tablespoon cookie to scoop roughly 16 cookies. Since the dough is sticky, I do recommend using a cookie scoop if you have one! Place on cookie sheets 2" apart.
Bake cookies one sheet at a time until the cookies are lightly crackled on top and the edges are turning golden, about 10-12 minutes. Don't overbake.
Cookies will be puffy and then flatten as they cool. Cool on the cookie sheets for about 10 minutes and then transfer to a cooling rack to cool completely.