Preheat oven to 300°F. Grease a 9 inch square baking pan.
Add brown sugar and butter to a medium pot over medium heat. Stir until combined. Add baking soda and continue to stir for a minute or so until mixture has consistency of honey. Remove from heat and add rolled oats and vanilla. Stir.
Press into baking pan. Bake for 20 minutes. Cut into squares while still warm.