Prepare 2 baking sheet pans by lining with parchment paper or silicone baking mats.
In a medium bowl, whisk together flour, baking soda and salt. Set aside for later.
1½ Cups All-purpose flour , 1½ teaspoon Baking soda, ½ teaspoon Salt
Using a stand mixer with the paddle attachment, begin mixing the butter, granulated sugar and brown sugar together until light and creamy (2 - 3 minutes on medium speed). Scrape down sides of bowl as needed. Note - a hand mixer may also be used.
1 Cup Unsalted butter, ½ Cup Granulated sugar, 1 Cup Brown sugar
Add eggs and vanilla extract to mixing bowl. Mix on medium speed until combined, scraping down sides of bowl as needed.
2 Large Eggs, 2 teaspoons Pure vanilla extract
Add flour mixture to mixing bowl. Mix on medium speed until combined, scraping down sides of bowl as needed.
Add oatmeal to mixing bowl and stir briefly until evenly distributed.
3 Cups Old-fashioned rolled oats
Remove the bowl from the mixer and stir in the M&Ms, peanuts, and raisins until evenly distributed. The dough will be a bit stiff at this stage, so expect to use a little extra arm power to mix everything in thoroughly
⅔ Cup M&Ms, ⅔ Cup Peanuts, ⅔ Cup Raisins
Form the dough into a disc, wrap it tightly in plastic wrap, and refrigerate for 30 minutes. This helps chill the dough for easier rolling and gives the flavors time to blend together.