Preheat oven to 350℉. Prepare a cookie sheet with parchment paper, if desired.
In the bowl of a stand mixer or with a hand mixer, cream together the butter and sugars on medium high for about one minute.
Once the butter and sugar are fluffy, add the prepared flax egg and vanilla. Mix on medium to combine.
Add the flour, baking soda, and salt. Mix on low until just combined. Add the oats and mix on low until combined.
Remove bowl from mixer, add mix-ins, and fold to combine.
Scoop heaping tablespoons from the mixture, roll gently, and add to the baking sheet about two inches apart.
Press down slightly on the dough balls to form a thick cookie shape (the cookies will rise, but won’t spread much in the oven). Top cookies with additional mix-ins, if desired
Bake the cookies at 350℉ for 10-12 minutes or until edges start to turn slightly golden.
Once cookies are finished baking, remove from the oven and let sit for at least five minutes before enjoying!