Use an electric mixer to cream together the butter and brown sugar until fluffy.
Add peanut butter, an egg, and vanilla. Continue to mix until smooth.
Next, add cornstarch, baking soda, salt, and flour. Mix until just combined. Pour in the quick oats, chocolate chips, and chocolate candies, and continue to mix.
Chill dough for at least 30 minutes. Roll about 2 tablespoons of dough into a ball and gently press down in the center. Add more candy to the tops, and bake for 11-13 minutes at 350°F.
Allow cookies to cool on the baking sheet before transferring them to a cooling rack. Store in an airtight container for up to 5 days.
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