Line the base of a 18cm cake tin with nonstick baking paper and grease the sides.
Heat the oven to 180C (160C fan-assisted)/350F/gas mark 4.
In a large pan, put 200g of the dates, tamarind paste and 300ml water and bring to the boil.
Boil for a minute, remove from the heat, add the butter, and set aside to cool down for about 5 minutes or until cool to the touch.
Add the brown sugar and eggs and beat the mixture until smooth. Stir in the flour, bicarbonate soda, vanilla and orange juice until well combined. Fold in the walnuts and spoon into the cake tin.
Bake for about an hour, or until a skewer inserted into the center comes out clean.
Meanwhile to make the butterscotch sauce, place all the ingredients into a small saucepan. Bring to a simmer and stir the ingredients until melted and combined. Simmer for about 5-7 minutes, stirring frequently, until the sauce reaches a dark color. Set aside and cool.
Cool in the tin for 10 minutes and then take the cake out of the cake tin and cool completely on a cake rack.
Spoon over the cooled butterscotch sauce, then scatter with the remaining chopped dates and walnuts.
Serve with the remaining butterscotch sauce.