Preheat your oven to 180°C (160°C fan). Grease and line a square baking tin with parchment paper.
In a large mixing bowl, cream together the softened butter and caster sugar until light and fluffy.
Beat in the eggs, one at a time, then stir in the vanilla extract.
Sift in the plain flour, baking powder, and salt. Gently fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix.
Spread the blondie batter evenly into the prepared baking tin, smoothing the top with a spatula.
In a small bowl, mix together the cinnamon and remaining caster sugar. Sprinkle this mixture evenly over the top of the blondie batter.
Allow the batter to rest for 30 minutes to proof slightly.
Bake in the preheated oven for 25 minutes or until the top is golden brown and a toothpick inserted into the centre comes out clean.
Once baked, remove from the oven and allow the blondies to cool completely in the tin.
Mix the remaining sugar and cinnamon and sprinkle over the top before slicing into squares.
