To the mixing bowl of a stand mixer fitted with the paddle attachment (or a mixing bowl using a hand mixer), combine peanut butter, brown sugar, egg, vanilla, and beat on medium-high speed until well-combined and the sugar is fully incorporated and is mixture is no longer gritty or granular, about 4 to 5 minutes. Stop to scrape down the bowl as necessary. (Regarding peanut butter – although natural peanut butter or homemade peanut butter may work, I recommend using storebought peanut butter like Jif, Skippy, Peter Pan or similar so that cookies bake up thicker and spread less. Using natural or homemade peanut butter tends to result in thinner and flatter cookies that are prone to spreading while baking)
Add the baking soda, optional salt, oats, and beat to incorporate. Add the 1 heaping cup pre-made trail mix OR add the chocolate chips, raisins, M&Ms and beat to just incorporate. Using a 2-inch medium cookie scoop (about 2 tablespoons), form 16 dough mounds. Squeeze and compact the dough mounds so all the add-ins stay in place. They will be prone to falling out because they’re abundant and dough is oily, just keep pushing them back in. Place dough mounds on a large plate. Flatten mounds slightly with your finger. Cover plate with plasticwrap and refrigerate for at least 2 hours, and up to 5 days, before baking. Do not bake with warm dough; cookies will spread and bake thin and flat.
Preheat oven to 350F, line 2 baking sheets with Silpat Non-Stick Baking Mats, parchment, or spray with cooking spray. Place mounds on baking sheets, spaced about 2 inches apart. I bake 8 to a tray. Bake for 10 to 12 minutes, until edges are set and tops are barely set, even if slightly underbaked in the center. Allow cookies to cool on trays for about 5 minutes before transferring to a wire rack to finish cooling. Cookies may not appear to be done at 10 to 12 minutes, but they likely are, and they firm up as they cool. Baking longer results in cookies with dark or burnt bottoms and that set up too crisp and hard and don’t keep well.
Store cookies in an airtight container at room temperature for up to 1 week, or in the freezer for up to 3 months. Alternatively, unbaked cookie dough can be stored airtight in the refrigerator for up to 5 days, so consider baking only as many cookies as desired and save the remaining dough to be baked in the future when desired.
Take care all ingredients used, such as the oats or any add-ins, are gluten-free and suitable for your dietary needs.