Preheat the oven to 350 degrees F. Grease and flour a 9 x 13" pan.
Toast the chopped nuts in a dry skillet until lightly toasted and aromatic.
Melt the butter in a microwave safe bowl. Add the water and cocoa, stirring well. Sift together the flour, sugar, and salt, and stir them into the chocolate mixture. In a large bowl, combine the eggs, buttermilk, vanilla, and baking soda. Add this mixture to the chocolate mixture and stir well.
Pour the cake into a prepared pan and bake at 350 degrees F for approximately 40 minutes or until a toothpick inserted in the middle comes out clean.
As the cake cools, make the icing. Melt the butter with the cocoa in a microwave safe bowl. Add the Half and Half, stir, and heat for about 30 seconds longer. Mix in remaining ingredients, blending well. Pour the icing over the cake in its pan while both are still warm.
Serve the cake warm or at room temperature.
Recipe by Baking and Eggs at https://bakingandeggs.com/2017/08/04/texas-sheet-cake/