Preheat your oven to 350°F first.
Grease your sheet pan with butter generously.
You can also use cooking spray instead.
Combine butter, water, and cocoa in saucepan.
Bring this mixture to a boil quickly.
Remove from heat immediately after boiling.
Whisk flour, sugar, baking soda, and salt.
Put everything in your large mixing bowl.
Make sure there are no lumps present.
Pour hot chocolate mixture over dry ingredients.
Stir until everything is well combined together.
Add eggs, sour cream, and vanilla next.
Mix until the batter is completely smooth.
Pour batter into your prepared sheet pan.
Spread it evenly across the entire surface.
Bake for 20 minutes until done completely.
Start frosting when cake almost finishes baking.
Melt butter in a saucepan over heat.
Add cocoa powder and milk to it.
Bring mixture to a gentle boil quickly.
Remove from heat right away after boiling.
Stir in powdered sugar and vanilla extract.
Add pecans if you're using them today.
Pour frosting over the still-warm cake immediately.
Spread it evenly with your clean spatula.
The warmth helps frosting soak in beautifully.
Let cake cool before cutting into pieces.