Preheat oven to 160°C (320°F) conventional. Grease and line a 20 or 22cm square cake tin.
Place sultanas and water in a large saucepan and bring to the boil over high heat. Boil for 8 minutes until sultanas are plump and most of the liquid has been absorbed.
Remove from the heat and add the cubes of butter. Stir through until all the butter is melted. Add the sugar and stir in well. Set mixture aside to cool for 20-30 minutes. If you want the mixture to cool quickly, place the base of the saucepan into a sink filled with some iced water.
While the mixture is cooling, sift the flour and baking powder together in a bowl. Once the mixture has cooled down, add the sifted flour and baking powder and stir together with a large spoon. Add the lightly beaten eggs and vanilla extract and continue to stir until everything is combined and there are no flour pockets left in the batter.
Pour the batter into the prepared baking tin. Bake for 1hr 10 minutes to 1 hr 25 minutes or until the middle is cooked and a skewer inserted into the centre of the cake comes out clean. You will need to cover the top of the cake loosely with foil half-way through the baking time once the top is nice and golden (at the 35-45 minute mark), otherwise it will darken too much. Once cooked, remove from the oven and allow to cool in the tin for 15-20 minutes. Invert onto a wire cooling rack to continue to cool completely.
Once cooled, cover the cake with cling wrap and place in an airtight container. Cake will keep for 5 days. This cake can also be frozen for up to 1 month - wrap well in cling film, then wrap again in aluminium foil and place in an airtight container or sealed glad bag and place in the freezer.