Preheat oven to 180℃.
Grease and line a 20cm (8") square baking tin with baking/parchment paper and set aside.
In a medium bowl, place plain flour and baking powder and whisk to combine, then set aside.
170 g plain flour (all-purpose flour), 1 teaspoon baking powder
Into a large bowl, place melted butter, light brown sugar, dark brown sugar and vanilla, then mix to combine.
115 g salted butter, 150 g light brown sugar, 70 g dark brown sugar, 2 teaspoons vanilla extract
Mix in the egg.
1 egg
Add in chopped dates and chopped walnuts, mixing to combine.
130 g dates, 60 g walnuts
Add in flour mixture and mix until just combined.
Pour the mixture into the prepared pan, pushing it into the corners and roughly smoothing the top.
Bake for 27 - 30 minutes, or until a deep golden brown.
Allow to cool in the tin for 10 minutes, then remove from the tin and allow to cool on a wire rack before slicing.