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Heat the oven to 180C/160C fan/gas 4. Oil and line a traybake tin, about 20 x 30cm, and 5cm deep. Put the oil, sugar, eggs and milk in a bowl and whisk until well combined. Sieve over the flour, cocoa and bicarb, and stir briefly until combined. Pour the mixture into the tin, and bake for 20-25 mins until the cake is well risen and springs back when pressed. Transfer to a wire rack and leave to cool completely.
Melt the chocolate in a bowl set over a pan of simmering water, ensuring the bowl doesn't touch the water, or in short blasts in the microwave until melted. Leave to cool for a few minutes.
Beat the butter and icing sugar together until pale and fluffy, then drizzle in the chocolate and beat again until smooth and uniform in colour. Swirl the icing over the top of the cake, with a few peaks and swirls to decorate. Scatter with the Easter treats to serve.