Preheat your oven to 350°F (175°C). Lightly grease your sheet pan or line it with parchment paper. I personally prefer greasing because it gives the edges a slightly crisp texture.

In a large mixing bowl, combine the flour, sugar, and salt. Give it a quick whisk so everything is evenly distributed.

In a saucepan over medium heat, melt the butter. Add water and cocoa powder, whisking until smooth and glossy. Bring it just to a gentle boil—this step helps bloom the cocoa, enhancing its flavor.

Carefully pour the hot chocolate mixture into the dry ingredients. Stir gently until combined. Add buttermilk, eggs, baking soda, and vanilla extract. Mix until smooth but avoid overmixing.

Pour the batter into your prepared pan and spread evenly. Bake for 18–20 minutes. The cake cooks quickly because it’s thin, so keep an eye on it.

While the cake bakes, prepare the frosting. Melt butter in a saucepan, then stir in cocoa powder and milk. Bring to a gentle boil, then remove from heat and whisk in vanilla and powdered sugar until silky smooth.

As soon as the cake comes out of the oven, pour the warm frosting over it. Spread quickly so it covers every inch. Sprinkle pecans on top if using.

Let the cake cool slightly before slicing. It’s delicious warm, but also amazing at room temperature.