Preheat the oven to 220C/425F/Gas 7.
For the scones, place the flour, baking powder and salt into a food processor with the butter and caster sugar. Blend until the mixture resembles breadcrumbs. Add the milk and blend until the mixture forms a dough.
Turn out the dough onto a floured surface and knead until the mixture is smooth. Roll or pat into a block 2.5cm/1in thick. Using a floured round cutter, cut into five scones.
Press a chestnut into the top of each scone and brush with a little milk.
Place the scones onto a non-stick baking sheet and bake for 15 minutes, or until well-risen and golden-brown.
For the sauce, melt the chocolate with the double cream in a mixing bowl set over a pan of just-simmering water (make sure that the bowl does not touch the surface of the water). Stir until melted and well-combined.
To serve, dust the scones with icing sugar and drizzle over the hot chocolate sauce.