Preheat oven to 180°C bake. Line a 20cm square tin with baking paper so it comes up the sides.
Melt butter and set aside for 5-10 minutes, until slightly cooled.
Meanwhile, zest & juice both lemons into two bowls – keeping zest and juice separate.
Place Lemon Blondie Mix into a large bowl. Add first measure of lemon zest and juice, eggs, vanilla, and cooled melted butter to mixture. Stir until smooth.
Pour into prepared baking tin and bake for 20-25 minutes until lightly golden on top and a toothpick inserted into the middle comes out clean. Set aside to cool.
To make the icing, add icing sugar to a medium bowl with second measure of lemon zest and juice. Gradually add the juice from both lemons, stirring the icing until a smooth, easy to drizzle, consistency forms. If you don't have enough juice, add a splash of water. Pour over cooled base and place in the fridge until set.
Lift from the tin and cut into even squares. Store at room temperature for soft cakey bars, or the fridge for a slightly denser slice.