In your large mixing bowl, whisk together the sugar, flour, salt, and baking soda. Pour the warm chocolate mixture from the saucepan directly into the dry ingredients. Whisk it together until it’s just smooth and combined. It will be thick and glossy.
Now, in a small bowl, whisk the eggs, sour cream, and vanilla together until smooth. Add this mixture to the chocolate batter and whisk again until everything is completely combined and the batter is smooth. The sour cream is the secret to the cake’s incredible moisture.
Pour the batter into your prepared pan, spreading it evenly to the corners. Bake for 18-22 minutes. The cake is done when a toothpick inserted in the center comes out clean or with just a few moist crumbs. Do not overbake! Start the frosting while the cake is still in the oven.
Pro Tips for Best Results
Start the frosting 5 minutes before the cake is done. This way, it’s ready to pour the second the cake comes out of the oven. The timing is everything—the warm, porous cake absorbs the frosting, making it incredibly moist and creating that signature gooey layer.
Use a good-quality cocoa powder. This is a chocolate-forward cake, so the cocoa flavor matters. I use a natural unsweetened cocoa for a classic taste. Don’t use Dutch-process here; the recipe relies on the acid in natural cocoa to react with the baking soda.
Don’t skip the sour cream. I tried it with yogurt once, and the texture wasn’t as rich or tender. The full-fat sour cream adds a subtle tang and unparalleled moisture that makes this cake stand out from other sheet cakes.
Toast your pecans for the frosting. Just spread them on a baking sheet and toast at 350°F for 5-7 minutes until fragrant. It deepens their flavor and adds a fantastic crunch against the soft cake and gooey caramel.
Common Mistakes to Avoid
Don’t overbake the cake. It continues to cook a bit from residual heat, especially when you pour the hot frosting over it. If you bake it until it’s completely dry, it can become dry overall. Pull it when it’s just set.(See the next page below to continue…)