Preheat the oven to 350°F. Spray an 18x13-inch rimmed sheet pan with baking spray.
In a medium saucepan, combine the water, butter, and cocoa powder. Bring to a boil over medium heat, stirring until smooth. Remove from the heat.
In a large bowl, whisk together the flour, sugar, baking soda, and salt. In a small bowl, whisk together the sour cream and eggs. Add the sour cream mixture to the flour mixture and beat with an electric mixer on medium speed until combined. (You can use a wooden spoon too, but the mixture will be stiff so I like to use a hand mixer.)
Add the warm butter mixture to the batter and carefully beat together on the lowest speed until well blended and smooth.
Pour the batter into the prepared sheet pan and smooth into an even layer.
Bake for 15 to 18 minutes or until a wooden pick inserted in the center comes out with a few moist crumbs.
While the cake is baking, make the frosting.
For the Frosting:
In a medium saucepan, combine the butter, milk, and cocoa.
Bring to a boil over medium-high, stirring until smooth. Remove from the heat. (If your cake is still baking, keep this mixture warm and proceed immediately after removing the cake from the oven.)
Add the powdered sugar and whisk until smooth. Stir in the pecans, if using. Immediately pour over the hot cake, quickly spreading it in an even layer over the cake. Let cool completely before slicing. Store the cake covered at room temperature for up to 5 days.
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Notes
I highly recommend using a scale to measure your flour as it’s the most accurate method and will prevent you from ending up with a dense sheet cake. If you don’t have a scale, fluff your flour with a spoon and spoon it into your cups before leveling it off with a knife. This method prevents you from overpacking the measuring cup.
Do not overmix the batter, or you’ll have a tough cake. Mix everything until just combined.
I recommend bringing the eggs to room temperature, so they incorporate into the batter without overbeating. If you forgot to bring the eggs out, place them in a bowl of warm tap water to help warm them up.
The cake needs to be hot when pouring on hot frosting, or the frosting will seize up when trying to spread. This is not a recipe you can divide into two prep days. However, the frosted and cooled cake will be great if made a day in advance.
Instead of mixing the pecans into the frosting, you can sprinkle them on top of the cake after covering with frosting.
I recommend you use a sheet pan with the highest rim you can find, so the frosting doesn’t go over the sides.
If the frosting sets up too quickly while you add in the powdered sugar, place the saucepan over low heat and whisk vigorously.
You can tap the sheet pan gently on the counter before baking to poke any air bubbles.