Dutch-processed cocoa gives this cake even more chocolate flavor. We’ll “choc” that up as a win.
Adjust oven rack to middle position and heat oven to 325 degrees. Spray bottom and sides of 13-by-9-inch metal baking pan with vegetable oil spray.
In medium bowl, whisk together sugar, flour, baking soda, and salt.
In large saucepan, combine chocolate chips, milk, and cocoa. Place saucepan over low heat and cook, whisking often, until chocolate chips are melted and mixture is smooth, about 5 minutes.
Add oil, eggs, and vanilla to saucepan with chocolate mixture and whisk until smooth, about 30 seconds.
Use rubber spatula to scrape batter into greased baking pan and smooth top (ask an adult for help because saucepan will be heavy).
Place baking pan in oven. Bake until toothpick inserted in center comes out with few crumbs attached, 30 to 35 minutes.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and let cake cool completely in pan, about 2 hours.
Once the cake is frosted, try any one of these tips to make it look extra-special: