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Heat the oven to 180C/160C fan/gas 4. Oil a 23 x 33cm baking tin (at least 4cm deep) and line the bases and sides with baking parchment. Whisk the milk and vinegar together in a jug – the milk should curdle slightly. Set aside.
Whisk the sugar, oil and vanilla extract together in a bowl, then whisk in the yogurt, making sure to break down any sugar lumps. Pour in the soured milk and mix well.
Sift the flour, cocoa powder, baking powder, bicarbonate of soda and ¼ tsp salt into a separate bowl and stir well to combine. Gradually whisk the wet ingredients into the dry until you have a smooth batter, but be careful not to over-mix.
Pour the batter into the tin and bake for 30-35 mins, until well risen and springy, and a skewer inserted into the centre comes out almost clean. A few sticky crumbs are fine, but the mixture should not be wet.
Leave the cake to cool in its tin for 20 mins, then carefully turn out onto a wire rack to cool completely. It will be delicate so be gentle. The sponge will keep, covered, at room temperature for up to two days.
For the vegan buttercream, melt the chocolate in the microwave or a bowl set over a pan of simmering water. Leave to cool. Beat the spread and vanilla on high speed in a stand mixer for a few minutes until pale and fluffy. Add the icing sugar gradually, beating on slow to start with, then turning up the speed to max until the mixture is light and creamy. Pour in the cooled chocolate and combine thoroughly, add a splash of plant milk if the icing is very stiff. Chill the buttercream for at least 30 mins before using.
When the cake is completely cool, spread the icing over the top and decorate with the sprinkles, if you like. Keep the cake in the fridge to stay firm, then remove 1 hr before serving. Will keep, covered, in the fridge, for up to three days.