Preheat oven to 350 degrees F. Grease a 10x15-inch jelly roll pan.
Make the Cake: In a medium saucepan, melt the butter over low heat. Add the water and cocoa powder, increase the heat to medium, and whisk until the mixture reaches a bowl. Remove from the heat and set aside.
In a large bowl, whisk together the flour, sugar, baking soda, and salt. Make a well in the center, add the eggs, sour cream, and vanilla, and whisk them together. Add the chocolate mixture and gently whisk to combine all ingredients thoroughly.
Pour into the prepared pan and bake until a toothpick comes out with moist crumbs attached, 18 to 22 minutes.
Make the Frosting: In a medium saucepan, melt the butter over low heat. Add the milk and cocoa powder and bring to a boil. Remove from the heat, whisk in the vanilla and powdered sugar until smooth. If using, stir in the chopped nuts.
Immediately pour the frosting over the warm cake, using an offset spatula to gently spread it evenly over the surface. Allow to cool for at least 30 minutes so the frosting can set, then serve warm or at room temperature.
Cocoa Powder: You can use your favorite cocoa powder in this recipe. I always reach for my regular unsweetened cocoa powder, but Dutch-process cocoa would work, too.
Sour cream: You can substitute buttermilk or plain yogurt for the sour cream.
Nuts: Traditional Texas sheet cake contains chopped pecans, but walnuts are also a popular option. You can omit them entirely, as well.
Storage: The cake should be covered and will keep at room temperature for up to 4 days, but can also be refrigerated (covered) for up to 1 week.
Freezing Instructions: This cake freezes very well! Once you frost it and let it cool completely, wrap it tightly in plastic wrap, then in aluminum foil and freeze for up to 1 month.