• Set the oven rack in the center position and preheat the oven to 350°F (176°C). Lightly grease a 9" x 13" baking dish with cooking spray or butter.
• Mix the dry ingredients in a small bowl and set aside.
  2 ½ cups all-purpose flour, 1 teaspoon salt, 1 teaspoon baking powder, 1 teaspoon baking soda
• Using a stand mixer or an electric hand mixer, beat the butter and sugar together until light and fluffy, about 3-4 minutes.
  1 ¾ cups granulated sugar, 8 tablespoons unsalted butter
• Add the eggs, milk, vanilla extract, and sourdough starter and continue to beat until smooth. Add the flour mixture to the wet ingredients and mix until just combined and there are no visible lumps of flour.
  3 large eggs, 1 cup whole milk, 1 ½ tablespoons vanilla extract, ½ cup sourdough starter discard
• Pour the batter into the baking dish and gently tap it onto the countertop to release any bubbles. Bake for 35-40 minutes or until a toothpick inserted into the center comes out clean.

Chocolate Frosting

• Using a handheld or stand mixer with a paddle or whisk attachment, beat the butter on medium speed for about 2 minutes until creamy. Add the confectioners' sugar, cocoa powder, heavy cream, salt, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 minute. If the frosting is too thin, add 2-3 tablespoons more confectioners' sugar or cocoa powder. If it's too thick, add another tablespoon of cream.
  1 cup unsalted butter, 3 ½ cups powdered sugar, ½ cup cacao powder, 3 tablespoons heavy cream, 1 tablespoon vanilla extract, pinch of salt