Method
Preparation
Preheat your oven to 350°F (175°C). Prepare an 18x13-inch sheet pan by greasing and flouring it to prevent sticking.
Whisk together the dry ingredients in a large mixing bowl: flour, sugar, baking soda, and salt.
In a saucepan, mix the butter, water, and cocoa powder. Bring it to a gentle boil over medium heat, stirring frequently.
Pour the hot mixture into your dry ingredients, mixing until just combined.
Add eggs, vanilla extract, and sour cream into the batter and mix until smooth.
Pour the cake batter into the prepared sheet pan, spreading it evenly.
Place the pan in your preheated oven and bake for 20–25 minutes, or until a toothpick inserted comes out clean.
Melt the butter for the frosting in a saucepan over medium heat. Stir in cocoa powder and milk, bringing this mixture to a boil.
Remove frosting from heat and whisk in the powdered sugar and vanilla extract until smooth and glossy.
Carefully remove the cake from the oven and pour the warm frosting over the hot cake, spreading it evenly.
Allow the cake to cool before slicing it into squares. Serve warm or at room temperature.
Notes
For enhanced chocolate flavor, use high-quality cocoa powder. Avoid overmixing for a light texture.