Preheat oven to 350˚F. Grease and line a 9×9-inch pan with aluminum foil or parchment paper. Set aside.
In the bowl of a stand mixer or using an electric hand mixer, beat together the butter and brown sugar. Once well combined, about 3 minutes, add the egg and vanilla. Finally, add the salt and flour.
By hand, stir in whatever additions you are using. In this recipe I am a using some chocolate chips, coconut and raisins, but feel free to add whatever you have on hand. Nuts would work well too. Pour the batter into your prepared pan. Sprinkle with some sea salt flakes. Not necessary but a delicious touch!
Bake in the oven for 20-25 minutes. Remove from the oven and allow to cool for about 5 minutes before transferring to a wire rack. I think these bars are best served while warm.