Preheat your oven to 350°F. Grease and flour a 9x13 pan, or a 15x10x2 inch sheet cake pan. Set aside.
Place the butter, water, and cocoa into a medium saucepan. Bring to a boil, stirring, then remove from the heat. Let this mixture cool slightly while you prepare the next steps.
In a large bowl, whisk together the flour, granulated sugar, cinnamon and salt.
In a large liquid measuring cup, whisk together the buttermilk, baking soda, vanilla, and eggs. Beat until well blended. Pour into the flour mixture and combine thoroughly.
Pour the hot butter mixture from the pot into the large bowl, mixing into the batter completely.
Pour the batter into the prepared pan, spreading evenly.
Bake for 22 to 25 minutes or until a toothpick inserted in the center comes out clean. The 9x13 pan will take slightly longer to bake.
Remove the cake from the oven. Keep it warm so don't chill it.
Prepare the icing. In a medium saucepan, heat the butter, cocoa, and milk until the mixture boils. Remove from the heat. Beat in the powdered sugar and vanilla with a whisk until the icing is smooth.
Pour the frosting over the warm cake. BRIEFLY place the cake into the fridge, to let the icing set and the cake cool completely. Remove it as soon as the icing is solid and slice to serve.
Notes
The hint of cinnamon in this is what makes this a Texas sheet cake more than a plain sheet cake in my opinion. It gives it a unique flavor that when paired with the frosting is exactly perfect.