Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease an approx 20 x 24 cm baking dish.
Mix together the light brown soft sugar and the cocoa powder for the sauce, set aside.
To make the sponge batter, sift together the flour, cocoa powder, baking powder and espresso powder in a bowl. Stir in the caster sugar and a pinch of salt.
Whisk together the milk, melted butter and vanilla extract.
Pour the wet ingredients into the dry and stir to form a smooth, thick batter.
Scrape the batter into the prepared baking dish and spread it level.
Sprinkle the sugar cocoa powder mixture evenly over the top. Gently smooth it out with the back of a spoon if needed.
Gently drizzle the boiling water over the back of a spoon over the top of the pudding. Do not stir it in.
Bake for 30 minutes until the cake is set and a skewer inserted into the centre of the cake section only comes out clean. (If you go too deep you will hit sauce).
Allow to cool for 5-10 minutes before serving straight from the dish.