Melt butter in a microwave-safe bowl. Add sugar and stir until combined. Add beaten egg and mix well.
Stir in oats, flour, vanilla, salt, and baking powder. Mix until combined.
Drop teaspoonfuls of dough onto the prepared baking sheet leaving space for the cookies to spread (we put 6 cookies on a sheet).
Bake for 8-9 minutes, or until the edges of the cookies are browned.
Let cool completely before removing from parchment paper.