Whisk together the butter, brown sugar and egg until paler in colour. You can also use an electric mixer.
Add the vanilla essence, flour and salt and incorporate using a spatula.
Line a 20x20cm mould with baking paper and pour half the blondie mix in. Level using a spatula and then spread out the cinnamon centre on top. Spread the second layer of blondie dough on top and sprinkle with the cinnamon sugar mix.
Take to a preheated oven at 180C for 15min