Combine ground flaxseed and water in a small bowl. Set aside for 10 minutes to let the mixture to gel and create your flax egg. Preheat the oven to 350℉. Line two large baking sheets with parchment paper or silicone baking mats.
Add the softened butter to the mixer or mixing bowl and cream for 2 to 3 minutes. Next, add the brown sugar and beat until fluffy, about another 2 to 3 minutes. Next, add in the peanut butter, vanilla, and flax egg, and mix until combined. Add the flax egg and vanilla extract. Cream together until combined.
Add all-purpose flour, rolled oats, baking soda, cinnamon, and salt to the bowl. Mix the ingredients together until the flour is evenly incorporated. Add M&M style candies and chocolate chips, and gently mix together.
Using a cookie scoop or spoon, scoop the cookie dough into about 12 cookie dough balls. Transfer cookie dough to the baking sheets.
Bake cookies for 15 minutes, or until the edges are light golden-brown and the center is still gooey. Remove cookies from the oven while they appear slightly underbaked. They will de-puff, continue to cook, and firm up while cooling. Let the cookies cool on the baking rack for 2 minutes before moving them to a cooling rack to fully cool. Allow cookies to cool before devouring. Enjoy!
Storage Instructions: Store Monster Cookies in an airtight container at room temperature for up to 4 days. For longer storage, freeze baked cookies for up to 3 months.