Preheat the oven to 170 degrees Celsius (340 Fahrenheit).
To make the pudding, add the dates to a small bowl and pour over 200 ml warm water - leave them to soak for 10 minutes.
In a large bowl, add the butter and light brown sugar and use a hand mixer to beat them together until creamy.
Add the egg and beat well.
Sift in the flour, baking powder and bicarb of soda, plus mixed spice, and mix everything well.
Add the mixed dried fruit and the dates chopped roughly together with their liquid, and mix again.
Butter and flour a 1 litre pudding basin, add the mixture, and bake for 50 minutes to one hour or until a skewer inserted in the middle comes out clean.
Leave it to cool for 10 minutes, then use a sharp knife to run it around the edges of the basin to get the pudding out.
Leave it to cool on a cooling rack.
To make the sauce, add the butter and sugar to a pan set over a medium heat.
Allow the butter to melt, then add the cream, vanilla extract and black treacle, and mix well.
Leave it to come to a boil, then remove it from the heat.
Pour some of the sauce over the pudding, the rest will be poured over the pudding as it's served.