This is my Best Moist Vanilla Sheet Cake that is just as easy to make with few simple ingredients and produces a very soft, tender and moist vanilla cake that is delicate yet doesn't crumble. It melts in your mouth and is still sturdy enough to stack and decorate.
Preheat the oven to 350°F. Line a 13x9-inch metal baking pan with parchment paper leaving a 2-inch overhang along the long sides for easy removal.
Combine dry ingredients. Use a fine mesh sieve to sift flour, baking powder and baking soda into a large bowl. Add sugar and salt and whisk to blend very well so the ingredients are evenly combined.
Combine wet ingredients. Add egg, egg whites, melted butter, oil, sour cream and vanilla to a medium bowl and whisk to blend well.
Warm the milk. Place cold milk in a heatproof jug and heat it in the microwave for 1 minute until steaming but not boiling.
Use a spatula to make a well in the center of the dry ingredients, then pour in the wet ingredients. Mix with an electric handheld mixer on medium-low speed until blended. You can also mix it in the bowl of a stand mixer fitted with the whisk attachment on medium speed or mix by hand using a wire whisk starting in the center and slowly dragging the dry ingredients in from the sides to combine evenly with the wet ingredients. It will be thick and somewhat dry. Add the hot milk gradually in two stages to minimize clumps forming and mix until evenly combined and the batter is smooth. The batter will be quite thin.
Bake. Pour batter into the prepared pan and tap the pan on the countertop to allow any large air bubbles to surface and pop. Bake for 23-27 minutes until the cake springs back when pressed gently and a skewer inserted into the center comes out clean. Transfer pan to a wire rack and let cool for 15 minutes before running a knife or offset spatula around the sides and transferring the cake onto the rack to cool completely.