Preheat oven to 170°C fan-forced.
Grease and line a 20 x 30cm baking dish with baking paper, leaving some overhang on the long sides.
Beat the butter and brown sugar until pale and creamy.
Add the eggs one at a time, beating well after each addition.
Mix in the vanilla extract.
Fold through the self-raising flour and salt until just combined.
Add the date mixture, including all the soaking liquid, and stir until evenly incorporated.
Pour the batter into the prepared baking dish and smooth the top.
Bake for 35–42 minutes, or until the centre springs back lightly when touched and a skewer inserted into the middle comes out with a few moist crumbs attached.