Preheat the oven to 375 degrees F (190 degrees C). Butter a 10x15 inch jellyroll pan.
Melt butter in a large saucepan over medium heat. Stir in brown sugar and vanilla. Remove from heat, and stir in oats, sesame seeds and baking powder until well blended. Press evenly into the prepared pan.
Bake for 7 to 10 minutes in the preheated oven, or golden brown. Cool in the pan over a wire rack. Cut into bars.