Preheat the oven to 180°C (350°F) and line a 5” square baking pan with baking paper and set aside.
Melt the butter and condensed milk together on medium low heat. Once melted, remove from heat immediately and whisk in the brown sugar and vanilla.
Sift in the dry ingredients (flour, cornflour and baking powder). Then fold until just combined (don’t overmix!).
Stir in the chocolate chips while the batter is still warm for a marbled effect.
Pour the batter into your prepped baking tin and then dollop on the Nutella, using a knife/ toothpick to make swirls across the batter. Don’t over-swirl!
Bake the blondies for around 15-16 minutes until the sides are firm but the centre is still soft. Don’t overbake if you want soft blondies.
Let the blondies cool down completely and then ENJOY! You can serve these warm or refrigerate for an even chewier texture.