Preheat oven to 325°F and line two large cookie sheets.
In a large bowl, or (preferably) the bowl of a stand mixer, beat together the butter and mint oil (see note) until fully softened.
Add the flour and icing sugar and, starting on low, begin to combine everything. Beat for 8-10 minutes (yes, really), gradually increasing the speed. It will go from crumbly to resembling fluffy buttercream.
In the last minute of beating, add a couple of tablespoons of sprinkles, if using, and the chocolate chips.
Scrape down the bowl and with a small disher or teaspoon, drop balls of dough onto the lined pans, a couple of inches apart. Top with remaining sprinkles, if desired. .
Bake one pan at a time for 12 minutes, or until the bottoms are just brown.
Let the cookies cool on the pan for a few minutes before carefully transferring them to a wire rack.
Store between sheets of wax paper in an airtight container for up to a couple of weeks.
NOTE – Make sure you use peppermint oil for this recipe and not extract. Oil is far more concentrated and will not add moisture (which would be detrimental to the outcome).