For the cake
For the frosting
Heat oven to 160C/140C fan/gas 3. Grease 2 x 20cm sandwich tins, then line the bases with baking parchment. Put the avocado and the sugar in a food processor and whizz until smooth.
Add the rest of the cake ingredients to the bowl with 1/2 tsp fine salt and process again to a velvety, liquid batter. Divide between the tins and bake for 25 mins or until fully risen and a skewer inserted into the middle of the cakes comes out clean.
Cool in the tins for 5 mins, then turn the cakes onto a rack to cool completely. While you wait, start preparing the frosting. Beat together the avocado and sunflower spread with electric beaters until creamy and smooth. Pass through a sieve and set aside.
Melt the chocolate, either over a bowl of water or in the microwave, then let it cool for a few mins. Sift the cocoa into a large bowl. Bring the soya milk to a simmer, then gradually beat into the cocoa until smooth. Cool for a few mins.
Tip in the avocado mix, icing sugar, melted chocolate and vanilla, and keep mixing to make a shiny, thick frosting. Use this to sandwich and top the cake.
Cover with sprinkles or your own decoration, then leave to set for 10 mins before slicing. Can be made 2 days ahead.