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Heat oven to 190C/fan 170C/gas 5 and make sure there’s a shelf ready in the middle of the oven. Butter a large loaf tin (about 20cm long, measured from the top edge), then cut a strip of baking paper a few inches longer than the total length of the base and the two shortest sides. Press it into the tin. (When your cake’s ready, you can use the paper ‘wings’ at either end to help lift it out.)
Put the butter and chocolate in a large saucepan and heat very gently until melted, stirring now and again. Cool for 5 mins, then use a whisk to mix in the yogurt and vanilla, then the eggs and a pinch of salt.
Mix the flour, sugar and bicarbonate of soda in a large bowl, squishing any big lumps of sugar with your fingers, then stir into the chocolate mix in the saucepan until even. Pour or spoon into the tin, then bake for 45 mins until risen and shiny (and probably with a nice crack along the length). Test if it’s ready by poking a skewer into the middle of the cake (see tip, below). Cool in the tin for 15 mins, then turn out and cool completely on a wire rack.
Put the cream into a small saucepan, bring to the boil, then add the butter and half the chopped chocolate. Take off the heat and leave to melt. Stir until smooth and shiny, then spoon over the cake. Scatter with the rest of the chopped chocolate and serve.