Put the dates in a small bowl and pour over 125ml of boiling water. Leave to cool for 30 mins. Lightly oil a 28 x 19 x 3cm traybake tin, then line the base with baking parchment.
Meanwhile, put the oil, black treacle, maple syrup and freshly grated ginger in a bowl and beat together with a fork to mix well. Set aside. Beat the egg in a small bowl and stir in the buttermilk. In a large bowl, mix the flour with the bicarbonate of soda, ground ginger, cinnamon and sugar – rub the mixture between your fingers to break down any lumpy bits of sugar. Heat oven to 160C/140C fan/gas 3.
Blend the dates and their liquid to a thick purée in a small food processor.
Pour the egg, the dates and the treacle mixture into the bowl with the flour. Stir together briefly with a wooden spoon just until well mixed – the mixture will be soft like a thick batter. Pour it into the lined tin, gently level the mixture and bake for 40-45 mins. To test if it’s done, insert a skewer in the centre – if the skewer comes out clean with no uncooked mixture on it, and the cake feels firm but springy to the touch, it should be done.
Leave in the tin for a few mins before removing to a wire rack, peeling off the parchment and leaving to cool completely. If you can wait, wrap it well in parchment, then foil. Leave for a day before cutting, as it will become stickier – it will keep moist for 3-4 days.