Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 15x10 inch jelly roll pan (or a similar-sized sheet pan).
Mix Dry Ingredients: In a large bowl, whisk together the flour, sugar, baking soda, and salt.
Prepare Cocoa Mixture: In a saucepan, melt the butter. Stir in the cocoa powder and boiling water, bringing it to a boil. Remove from heat and pour the mixture over the dry ingredients. Stir until combined.
Add Wet Ingredients: Add the buttermilk, beaten eggs, vanilla extract, and vinegar (if using) to the batter. Mix until smooth.
Bake the Cake: Pour the batter into the prepared pan and spread it evenly. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
Prepare the Frosting: While the cake is baking, make the frosting. In a saucepan, melt the butter, then stir in the cocoa powder and milk. Bring to a boil, then remove from heat. Add the powdered sugar and vanilla extract, mixing until smooth. If using, stir in the chopped pecans.
Frost the Cake: As soon as the cake comes out of the oven, pour the warm frosting over the top, spreading it evenly.
Cool and Serve: Allow the cake to cool slightly before slicing. Serve warm or at room temperature.
LIKE SHARE AND SAVE