Preheat the oven to 350 degrees F. Line a 9×13-inch baking pan with nonstick foil.
Beat together the butter and brown sugar on medium speed until light and fluffy, about 5 minutes. Add the eggs one at a time, and then the vanilla. Beat, scraping the bowl, until thoroughly combined.
Add the dry ingredients and the Heath pieces. Mix well.Spread the dough evenly into the pan (I found an offset spatula was the best tool for the job, as it’s a thick batter).
Mix sugar and cinnamon in a small bowl and sprinkle evenly over the top of the batter.
Bake for 25-30 minutes.
Cool completely before cutting.