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1
Preheat the oven to 350 degrees.
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2
Butter and flour a 12 by 18 by 1 1/2-inch sheet pan and set aside.
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3
In a medium bowl, sift together the flour, cocoa, baking soda, and salt.
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4
In the bowl of an electric mixer fitted with a paddle attachment, cream the butter and sugars on high speed until light, approximately 5 minutes.
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5
Add the eggs, one at a time, and the vanilla and mix well.
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6
Combine the buttermilk, sour cream, and coffee.
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7
On low speed, add the flour mixture and the buttermilk mixture alternately in thirds, beginning with the buttermilk mixture and ending with the flour mixture.
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8
Mix the batter only until blended.
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9
Pour the batter into the prepared pan and smooth the top with a spatula.
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10
Bake in the center of the oven for 25 to 30 minutes, or until a toothpick comes out clean.
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11
Cool to room temperature before frosting.
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12
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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13
Off the heat, add the corn syrup and vanilla and allow the chocolate mixture to cool to room temperature.
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14
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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15
Spread the frosting evenly on the cake.
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16
Have the children decorate the cake with M&M's.
