Serves: One cake1) Preheat the oven to 180C/Gas 4. Butter and flour a 30 by 45 by 4cm sheet pan.
2) Cream the butter and sugar on medium-high speed in the bowl of an electric mixer fitted with a paddle attachment until light and fluffy, about 5 mins. On medium speed, add the eggs, 2 at a time, then the sour cream, vanilla and lemon zest, scraping down the bowl as needed. Mix well.
3) Sift together the flour, cornflour, salt and bicarbonate of soda. With the mixer on low speed, slowly add the flour mixture to the butter mixture and stir just until smooth. Finish mixing by hand to be sure the batter is well mixed.
4) Pour the batter evenly into the pan, smooth the top with a spatula, and bake in the centre of the oven for 25 to 30 mins, or until a cocktail stick comes out clean. Cool in the pan to room temperature.
For the icing:
1) Place the chocolate chips and double cream in a bowl set over a pot of simmering water, stirring occasionally, until the chips are completely melted. Off the heat, add the golden syrup and vanilla and allow the chocolate mixture to cool to room temperature.
2) In the bowl of an electric mixer fitted with the whisk attachment, whisk the chocolate mixture and softened butter on medium speed for a few minutes, until it's thickened.
3) Spread the icing evenly on the cake. Allow the children to decorate the cake as they wish with chocolate sweets.