• Preheat oven to 350°F (177°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
• Mix dry ingredients: In a medium bowl, whisk together the cake flour, baking powder, and salt. Set aside.

  *If you don't have cake flour use the substitution: Measure 2 cups of all-purpose flour, remove 4 tablespoons, and replace with 4 tablespoons of cornstarch. Sift well.
• Cream butter and sugar: In a large bowl, whisk together melted butter and sugar until smooth. Add egg whites and vanilla extract, whisking until light and frothy.
• Alternate wet and dry ingredients: Add half of the dry ingredients, stir to combine, then mix in half of the milk. Repeat with the remaining dry ingredients and milk, stirring until just combined.
• Fold in sourdough discard until incorporated. Do not overmix.
• Bake: Pour batter into the prepared pan, smooth the top, and bake for 38-45 minutes, or until a toothpick inserted in the center comes out clean. Moist crumbs are just fine, but you don't want any wet batter on the toothpick.

  Oven time can vary greatly here, so make sure you're keeping an eye on the cake!
• Cool: Let cake cool for 10 minutes in the pan, then release the springform ring and cool completely on a wire rack before frosting.

Frosting Instructions

• Beat butter and cream cheese until smooth (about 2 minutes).
• Add vanilla and salt, then gradually mix in sifted powdered sugar one cup at a time.
• Beat until fluffy (about 2-3 minutes on medium-high speed).
• Frost the cooled cake and enjoy!