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1
Preheat the oven to 350 degrees F. Butter and flour a 12 by 18 by 1 1/2-inch sheet pan.
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2
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.
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3
On medium speed, add the eggs, 2 at a time, then the sour cream, vanilla, and lemon zest, scraping down the bowl as needed.
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4
Mix well.
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5
Sift together the flour, cornstarch, salt, and baking soda.
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6
With the mixer on low speed, slowly add the flour mixture to the butter mixture and stir just until smooth.
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7
Finish mixing by hand to be sure the batter is well mixed.
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8
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.
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9
Cool in the pan to room temperature.
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10
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.
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11
Off the heat, add the corn syrup and vanilla and allow the chocolate mixture to cool to room temperature.
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12
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.
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13
Spread the frosting evenly on the cake.
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14
Have the children decorate the cake with chocolate candies.
