http://relish.com/recipes/texas-sheet-cake/

Cooking spray
2 cups all-purpose flour
1 3/4 cups sugar
1 teaspoon baking soda
1/4 teaspoon salt
1 cup (2 sticks) butter, cut into small pieces
1 cup water
1/3 cup unsweetened cocoa
2 eggs
1/2 cup buttermilk
1 teaspoon vanilla extract

Frosting:
3 egg yolks
1 cup evaporated milk, divided
1 teaspoon vanilla extract
1 cup sugar
1/2 cup butter, cut into small pieces
1 1/3 cups flaked, sweetened coconut
1 cup chopped pecans

2 cup s all-purpose flour 1 3/4 cups sugar 1 teaspoon baking soda 1/4 teaspoon salt 1 cup (2 sticks) butter, cut into small pieces 1 cup water 1/3 cup unsweetened cocoa 2 eggs 1/2 cup buttermilk 1 teaspoon vanilla extract 3 egg yolks 1 cup evaporated milk, divided 1 teaspoon vanilla extract 1 cup sugar 1/2 cup butter, cut into small pieces 1 1/3 cups flaked, sweetened coconut 1 cup chopped pecans

Preheat oven to 350F. Coat a 13-by-9-by-2-inch baking pan with cooking spray.
To prepare the cake, sift flour, sugar, baking soda and salt into a large bowl.
In a small saucepan, combine butter, water and cocoa. Cook over medium heat, stirring occasionally, until butter melts and mixture comes to a simmer.
Add cocoa mixture, eggs, buttermilk and vanilla to flour mixture; stir to combine. Pour into pan and bake 30 minutes or until a wooden pick inserted in the middle comes out clean. Let cool on wire rack. Do not remove from pan.
To prepare the frosting, whisk egg yolks and about 1/3 cup milk in a saucepan. Add remaining milk, vanilla and sugar. Add butter; cook over medium heat, stirring with a wooden spoon, until it comes to a full boil. Reduce heat to maintain boil and cook, stirring frequently, 7 minutes or until thick. Remove from heat and stir in coconut and pecans. Cool, stirring occasionally, until thick enough to spread.
Spread frosting over top of cake.