Preheat oven to 375 degrees F. (if oven runs hot, use 350).**
Stir together flour, baking soda and salt. Set aside.
Cream butter and both sugars using high speed of an electric mixer, scraping sides of bowl often. Add egg and vanilla and beat for 30 seconds. Add flour mixture and stir until absorbed, then stir in oats and butterscotch chips.
Drop by generously rounded teaspoonfuls (¾ inch mound) onto ungreased cookie sheets. Alternatively, you can use a medium size cookie scoop for bigger cookies.
Flatten tops slightly and bake for 12 minutes or until golden brown around edges.
Notes
I think baking them at 375 helps the sugar caramelize a little more quickly and the edges are darker and chewier. If your oven runs hot or your cookies are coming out soft in the center and overdone around the edges, lower the heat to 350.