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.

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.