Start by preheating your oven to 350°F (175°C). Lightly grease a 12×18-inch sheet pan (or jelly roll pan) with nonstick baking spray to ensure the cake doesn’t stick.

In a large mixing bowl, combine the sugar, flour, and salt. Whisk these dry ingredients together and set the bowl aside.

In a medium-sized saucepan over medium heat, melt the butter completely. Once melted, stir in the cocoa powder, ensuring no lumps remain. Gradually pour in the hot water, stirring continuously. Allow the mixture to reach a gentle boil and let it bubble for about 30 seconds before removing the pan from the heat.

Carefully pour the warm chocolate mixture over the dry ingredients in the bowl. Mix everything thoroughly until the batter is smooth. Let this mixture cool for a couple of minutes.

Meanwhile, in a measuring cup, beat the buttermilk and eggs together until fully combined. Add the baking soda and vanilla extract to the buttermilk mixture and mix well.

Once ready, pour the buttermilk mixture into the chocolate batter. Whisk the ingredients together until you achieve a uniform, silky-smooth batter.

Evenly spread the batter into the prepared baking pan, smoothing out the surface with a spatula if needed. Place the pan into the oven and bake for 15-20 minutes, or until a toothpick inserted into the center comes out clean.

While the cake bakes, prepare the icing. In a clean saucepan, combine the butter, milk, and cocoa powder. Heat the mixture over medium heat until it reaches a rolling boil, stirring consistently to prevent burning. Once it boils, remove the saucepan from the heat immediately.

Gradually whisk in the powdered sugar and vanilla extract, ensuring the icing becomes velvety smooth and lump-free.

Once the cake is finished baking, remove it from the oven and pour the warm icing evenly over the hot cake. Use an offset spatula or the back of a spoon to spread the icing evenly across the surface.

Let the cake cool slightly so the icing can set before slicing and serving. Enjoy this indulgent, chocolatey Texas sheet cake warm or at room temperature!