Heat the oven to 180C/160C fan/gas 4. Oil the base and sides of a 34 x 25cm roasting or cake tin (at least 2½cm deep) and line with baking parchment. Combine the flour, cocoa, baking powder, bicarb, sugar and a good pinch of salt in a large bowl. Break up any lumps of sugar by rubbing between your fingers, shaking the bowl to bring them to the surface.
Whisk the oil, yogurt, cola and eggs together in a jug, then pour the wet ingredients into the dry. Stir well, ensuring no pockets of flour remain, then pour into the tin and bake for 30 mins until a skewer inserted into the centre comes out clean. If any wet mixture clings to the skewer, bake for 5 mins more. Cool in the tin for at least 20 mins.
For the icing, melt the butter in a pan over a low heat. Remove from the heat and stir in the sugar, milk and food colouring. Spread the icing over the cake, then decorate with the sprinkles, cherries and sweets, if using. Leave to set, then cut into squares. Will keep in an airtight container for up to five days.
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