Tex-Mex Chocolate Sheet Cake (with German Chocolate Variation)

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1. 1Step 1: Position a rack in the center of the oven and preheat the oven to 375°F. Spray the half sheet pan with baking spray.
2. 2Step 2: Make the cake: Place the 1 cup of butter, sour milk, 6 tablespoons cocoa, and 1 1/2 teaspoons cinnamon in a small saucepan over medium-low heat. Bring to just under a boil, stirring to melt the butter and blend the mixture.
3. 3Step 3: Beat the eggs with the water, 1 teaspoon vanilla, and baking soda in a small bowl.
4. 4Step 4: In a large bowl, whisk the flour, granulated sugar, and 1/2 teaspoon salt together.
5. 5Step 5: Add the egg mixture to the flour, mixing gently until well combined. Add the butter-cocoa mixture and stir well.
6. 6Step 6: Pour the batter into the prepared pan and lightly tap the pan down on the counter to make sure the batter is evenly distributed and to remove excess air bubbles.
7. 7Step 7: Bake until the cake pulls away from the sides of the pan and a toothpick inserted in the center comes out clean, 20 to 25 minutes.
8. 8Step 8: While the cake is baking, make the icing, as you will want to spread it on the cake while both are still warm. Place the 8 tablespoons butter, 6 tablespoons cocoa, and whole milk in a small saucepan over medium-low heat. Bring to just under a boil, stirring to melt the butter and blend the mixture.
9. 9Step 9: Remove the pan from the heat and add the 2 teaspoons vanilla.
10. 10Step 10: Whisk the confectioners' sugar, 1 teaspoon cinnamon, and a pinch of salt together in a separate bowl. Add the butter mixture to the sugar mixture and blend thoroughly.
11. 11Step 11: Spoon the warm icing over the warm cake 5 to 7 minutes after it comes out of the oven. Spread the icing using an offset spatula.
12. 12Step 12: Let the iced cake cool completely in the pan on a wire cooling rack before cutting into squares.
13. 13Step 13: For the German Chocolate Variation: Place the evaporated milk, sugar, egg yolks, butter, vanilla, and salt in a medium-size heavy saucepan. Mix well like you're mixing a batter. Place the saucepan over medium heat and cook, stirring continuously, until thickened, about 12 minutes. You may notice small particles in the mixture, but don't worry. You will strain them out.
14. 14Step 14: Remove the pan from the heat and pour the mixture through a fine sieve set over a large bowl. Stir the mixture vigorously with a fork so that it strains through the sieve. Work quickly so that the icing is still hot when you're finished.
15. 15Step 15: Mix in the coconut and chopped pecans. Spread the frosting on top of the warm chocolate sheet cake. Let cool completely in the pan on a wire cooling rack before cutting into squares.