For the cake:
18 Tablespoons (2-1/4 sticks) unsalted butter, at room temperature
3 cups sugar
6 extra-large eggs, at room temperature
8 ounces (about 1 cup) sour cream, at room temperature
1-1/2 teaspoons pure vanilla extract
Zest of 1 lemon
3 cups all-purpose flour
1/3 cup cornstarch
1 teaspoon kosher salt
1 teaspoon baking soda

For the Frosting:
24 ounces chocolate chips
1-1/2 cups heavy cream
2 Tablespoons light corn syrup
1/2 teaspoon pure vanilla extract
4 Tablespoons unsalted butter, at room temperature

M&M's candy for decorating