Preheat the oven to 350°F. Grease and flour an 11 x 17 x 1-inch sheet pan (an 18 x 13 x 1-inch pan also works).

In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

In a medium saucepan, melt the butter. Add the water and cocoa, then bring the mixture to a boil while stirring often.

Remove from heat and stir in the granulated sugar, sour cream, eggs, and vanilla.

Add the flour mixture to the saucepan and stir until the batter is smooth and well combined.

Pour the batter into the prepared pan and bake for 20 to 25 minutes, or until a toothpick inserted in the center comes out clean.

In a medium saucepan, heat the butter, cocoa, and milk. Bring to a gentle boil, stirring often.

Remove from heat and whisk in the vanilla and powdered sugar until smooth.

If using pecans, stir them in now.

Pour the icing over the hot cake and spread evenly.

Let the cake cool completely before cutting into squares.