Preheat the oven to 350 degrees F. Butter and flour a 12 by 18 by 1 1/2-inch sheet pan.
To make the cake, 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 minutes. 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. Sift together the flour, cornstarch, salt, and baking 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. Pour evenly into the pan, smooth the top with a spatula, and bake in the center of the oven for 25 to 30 minutes, or until a toothpick comes out clean. Cool in the pan to room temperature.
For the frosting, place the chocolate chips and heavy cream in a bowl set over a pot of simmering water, stirring occasionally, until the chips are completely melted. Off the heat, add the corn syrup and vanilla and allow the chocolate mixture to cool to room temperature. 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.
Spread the frosting evenly on the cake. Have the children decorate the cake with chocolate candies.
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