Preheat oven to 180°C / 350°F/ Gas 4. Line and grease a 20cm/ 8” round loose base cake tin.
Sieve the flour, cornflour, sugar, 3 tablespoons cocoa, baking powder, bicarbonate of soda and salt together and place in the food mixer. Add the water, oil, vinegar and vanilla. Mix well until smooth and pour batter into the prepared tin. Bake in preheated oven for 30 minutes before leaving to cool.
Meanwhile, combine the 45g cocoa powder, sweetened condensed milk and butter in the top half of a double boiler and cook until thick. Spread the warm mixture over the top of the slightly cooled cake.
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