Preheat the oven to 350 degrees. Butter and flour a 12 by 18 by 1 1/2-inch sheet pan and set aside.
In a medium bowl, sift together the flour, cocoa, baking soda, and salt.
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. Add the eggs, one at a time, and the vanilla and mix well. Combine the buttermilk, sour cream, and coffee. 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. Mix the batter only until blended.
Pour the batter into the prepared pan and smooth the top with a spatula. Bake in the center of the oven for 25 to 30 minutes, or until a toothpick comes out clean. Cool to room temperature before 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 M&M's.