Preheat oven to 350°F. Line a cookie sheet with parchment paper or foil. Remove peanut butter cups from jar; set aside. Empty the remaining contents of the jar into a large bowl. In another bowl, whisk together 1/2 cup creamy peanut butter, 1/4 cup softened butter, 2 eggs, and 1/2 teaspoon vanilla. Add to flour mixture; stir until combined. Gently stir in peanut butter cups. Use a 1/4-cup measure or scoop to drop mounds of dough about 4 inches apart onto prepared cookie sheet. Flatten dough mounds to about 3/4 inch thick. Bake for 12 to 14 minutes or until edges are brown. Cool on cookie sheet for 1 minute. Transfer to a wire rack and let cool. (For regular-size cookies, drop dough by rounded teaspoons 2 inches apart on cookie sheet. Bake for 9 to 11 minutes.)
Store jar of mix in a cool, dry place for up to 1 month.