Hayden's Chocolate Cake

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3 1/2 cups all-purpose flour

2 cups good cocoa powder

1 tablespoon baking soda

1/2 teaspoon kosher salt

1 1/2 cups (3 sticks) unsalted butter, at room temperature

1 1/3 cups light brown sugar, lightly packed

1 1/3 cups granulated sugar

4 extra-large eggs, at room temperature

4 teaspoons pure vanilla extract

2 cups buttermilk, at room temperature

1 cup sour cream, at room temperature

1/4 cup brewed coffee

Candy-coated chocolates, such as M&M¿s, for decorating

For the frosting:

24 ounces semisweet chocolate chips

1 1/2 cups heavy cream

2 tablespoons light corn syrup

1/2 teaspoon pure vanilla extract

4 tablespoons (1/2 stick) unsalted butter, at room temperature