Preheat the oven to 190°C (Gas Mark 5 / 375°F).
Add the melted butter, vanilla extract and eggs to a large mixing bowl, and whisk to combine. Add the caramel bits (if using) and diced apple, and mix again.
Next add the dry ingredients - the flour, brown sugar, baking powder and cinnamon. Mix until well combined, but try not to overmix.
Grease a brownie tray (mine measured 11 x 7 inches), and add the toffee apple blondie mixture. Spread the mixture out into an even layer, and bake for around 25 minutes, until the blondies are firm and golden brown, and an inserted toothpick comes out clean.
Prick the blondies all over with a toothpick, and pour over the toffee sauce. Spread the sauce around so that it coats the blondies. Leave to cool, then cut into squares and serve.