Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together the flour, sugar, baking soda, and salt. Set aside.
In a medium bowl, whisk together the eggs, milk, oil, and vanilla extract. Slowly add the wet ingredients to the dry ingredients while whisking until just combined. Whisk in the cocoa powder until combined.
Pour the batter into the prepared pan and spread evenly. Bake for 25-30 minutes, or until a toothpick comes out clean when inserted into the center.
Let the cake cool completely before frosting.
To make the frosting, whisk together the melted butter and cocoa powder in a medium bowl until combined. Add the confectioners' sugar, vanilla extract, and 1/3 cup of the milk. Whisk until combined, then add more milk if needed until desired consistency is reached.
Spread the frosting over the cooled cake and serve.
Interesting Facts
Sheet cakes are a type of cake baked in a large, flat rectangular pan.
Sheet cakes are usually frosted with a thin layer of frosting.
This type of cake is often served at potlucks or other large gatherings.
Sheet cakes are often decorated with colorful sprinkles or other decorations.