Preheat oven to 350F and line a 8X8 pan with foil and grease.
In a medium saucepan cook apple chunks in butter and brown sugar. Cook on medium heat for 4-6 minutes, until apples are slightly softened.
In a medium bowl mix butter and sugar together. Whisk in egg and vanilla extra. Mix in flour, baking soda and salt. Fold in all of apple mixture. Pour into prepared pan and bake for 35-40 minutes, until middle is set and golden brown. Place on wire rack and let cool completely before slicing.