Cake

Cup of Yum
1. Preheat oven to 375F and spray a 10x15x2-inch nonstick jelly roll pan very well with cooking spray or grease and flour the pan. Tip - The recipe is intended for this size pan. Read the FAQ section of the blog post if you have questions regarding using other sized pans.
2. To a large stockpot, add the butter or margarine, water, cocoa powder, turn the heat to medium-high, and bring the mixture to a boil for about 5 seconds. Whisk nearly constantly until the butter has melted and the cocoa powder has incorporated. After the mixture boils for a couple seconds, take it off the heat.
3. Off the heat, add the flour, granulated sugar, baking soda, salt, and whisk well to combine. Tip - If you're using margarine rather than butter, it's already salted so you may consider omitting the salt.
4. Add the eggs, sour cream, vanilla, and whisk to combine. Tip - Make sure the batter has cooled a bit before adding the eggs so they don't scramble inside the batter and so that the sour cream doesn't curdle. If you stick your finger in the batter, it shouldn't be so hot that you're practically burning yourself which shouldn't be the case after you whisked in other dry ingredients in the previous step, but it's worth mentioning.
5. Evenly pour the batter into the prepared pan, and bake on the center oven rack for about 23 to 25 minutes, or until done. Rotate pan once midway through baking for optimal results. Cake is done with the top is set, lightly springy to the touch, and a toothpick inserted into the center comes out clean, or with a few moist crumbs. Set the cake aside while you make the icing. Tip - You want to add the frosting when the cake is still warm, so hustle along and get it made - you can't make it too soon such as when the cake is baking because it will start to set up in the pot.

Frosting

1. To the same large stockpot (cleaned and dried), add the butter or margarine, cocoa powder, milk, turn the heat to medium-high, and bring the mixture until just boiling. Whisk nearly constantly until it boils. Remove the pot from the heat.
2. Slowly add the confectioners' sugar, whisking constantly as you add it to help encourage it to dissolve and incorporate. Make sure it's smooth and silky before moving on.
3. Add the vanilla and whisk to combine.
4. If you want to cut the overall sweetness of the frosting a bit, you can add the salt, although it's optional, but I add it.
5. Add the nuts and stir to combine.
6. Turn the icing out over the cake while both the cake and icing are still warm. Use a spatula to smooth it as necessary.
7. Allow the icing to set up for about 15 minutes (longer is fine), or as necessary before slicing and serving.