Melt 1 tablespoon of butter in a skillet or pan over medium heat. Add 2 tablespoons brown sugar and apple pieces. Stir to combine. Reduce heat to low and simmer for 5 minutes or until apples begin to soften. Remove from heat and let cool slightly.
In a medium bowl, melt the remaining butter. Stir in remaining brown sugar.
Whisk in egg and vanilla extract.
Stir in flour and salt. Mix until just combined. Fold in cooled apples. Pour into prepared baking dish.
Bake in oven for 30-40 minutes or until bars are set (a thoothpick inserted into the center comes out clean). Let cool completely before drizzling with caramel (if using) and cutting.