Preheat oven to 350 degrees F and line two baking sheets with parchment paper.
In a large bowl with an electric mixer, beat butter and sugar until light and fluffy (about 3-4 minutes).
Add egg and vanilla and beat until smooth.
Add oats, flour, cinnamon, baking powder, baking soda and salt and stir with an electric mixer or by hand until completely combined. Stir in butterscotch chips.
Drop by heaping tablespoonfuls (if you're using a cookie scoop, you'll want to use a 2 TBSP scoop) onto parchment paper 2 inches apart (I do 12 per pan). If you like thinner, chewier cookies, press down slightly (this step is optional).
Bake at 350 degrees F for 8-9 minutes, until the edges are beginning to brown and the centers are almost set (a tiny bit of glossiness in the center is okay, as they will continue to set on the hot pans).
Let sit for 5 minutes before removing from the pan to cool completely. Store in an air tight container up to 1 week, or freeze for up to 3 months.