Grease and line a 28 x 18 cm (11 x 7 inch) shallow baking tin with nonstick baking paper. Put 125 g (4 oz) of the dates in a saucepan with the water and bring to the boil. Reduce the heat and cook gently for 5 minutes or until the dates are pulpy. Turn into a bowl and leave to cool. Put the cream, muscovado sugar and 75 g (3 oz) of the butter in a small saucepan and heat gently until the sugar dissolves. Bring to the boil and boil for 5 minutes or until thickened and caramelized. Leave to cool.
Put the remaining butter in a bowl with the caster sugar, vanilla bean paste and eggs, sift in the flour and baking powder and beat with a hand-held electric whisk until pale and creamy. Beat in the cooked dates and 100 ml (3½ fl oz) of the caramel mixture. Turn into the tin and level the surface. Scatter with the remaining dates.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25 minutes, or until just firm. Spoon the remaining caramel on top and return to the oven for 15 minutes until the caramel has firmed. Transfer to a wire rack to cool.