Note: this dough needs to be chilled. Plan ahead!
In a medium bowl, whisk cinnamon, salt, baking soda, and flour. Set aside.
Cream butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. (You can also use a large bowl and a hand mixer.) Cream until smooth, about 1-2 minutes. Mix in eggs and vanilla, beat until smooth.
Gradually mix in dry ingredients until smooth, being careful not to over mix. Add oats and mix, then stir in desired mix-ins.
Scoop 2 tablespoon sized balls of dough and place on a cookie sheet lined with parchment or wax paper. Cover with plastic wrap and chill for at least 1 hour.
When ready to bake, preheat oven to 350°F. Line two cookie sheets with parchment paper or a silpat baking mat and place the cookie dough balls 2” apart.
Bake for 10-12 minutes, until the bottom is slightly golden. Cool on cookie sheets for 10 minutes, then remove to a rack to cool completely.
Store cookies in an airtight container for up to 4 days. They can also be frozen in an airtight container or ziploc bag for up to 2 months.