Preheat the oven to 180C/350F/gas 4. Line the base and sides of a deep-sided 20cm/8in round cake tin with a non-stick baking parchment. Sprinkle the base with the demerara sugar.
Place a cherry in the cavity of each pear half where the core has been removed then place the pears, cut side down, on the sugar in the cake tin, radiating from the centre of the tin.
Melt the butter, syrup and treacle together in a small pan over a low heat, then set aside. In a jug, mix together the egg, milk, and bicarbonate of soda.
Sift the flour, cinnamon and ginger into a mixing bowl and stir in the soft brown sugar. Make a well in the centre and pour in the melted ingredients and the egg and milk mixture. Stir thoroughly until well combined. Pour over the pears into the prepared cake tin.
Bake in the centre of the oven for about 45 minutes, or until well risen, golden-brown and firm to the touch. Cool in the tin for 3-4 minutes then turn out onto a serving plate, pear-side-up. Cut into thick slices and serve warm with vanilla ice cream.