Pre-heat oven to Gas 4, 180°C, fan 160°C.
Put paper cases into a metal bun tin.
In a large mixing bowl, sieve together the flour, baking powder and bicarbonate of soda. Stir in the sugar and mix in 100g (3½oz) butter until the butter is mixed through the flour. The mixture will resemble rough breadcrumbs at this stage.
In a separate measuring jug, whisk together the water, milk, eggs and vanilla extract and set aside.
Melt the chocolate in a small, heatproof bowl over a pan of hot but not boiling water. Make sure the water never touches the bottom of the bowl.
Once the chocolate has melted, make a well in the centre of the flour and pour in the egg mixture and melted chocolate. Beat with an electric hand mixer at low speed for 30 seconds to bring the mixture together, then at a high speed for a further 3 minutes.
Divide the mixture equally between the paper cases. Fill each case half to two thirds full.
Place in the oven and bake for 15-20 minutes. Leave them to cool on a rack.
To make the frosting, beat together the butter and icing sugar until smooth. Combine the cocoa powder and water in a separate small bowl and stir until all the lumps have dissolved. Add this to the mixture and beat until smooth and creamy. Top each cupcake with frosting and serve.
Please note: this recipe has been created and uploaded by a member of the Tesco Real Food online community. Tesco does not test customer recipes so cannot accept responsibility for the content, validate the results or answer queries regarding them.