Prep Pans & Oven: Preheat oven to 350°F (175°C). Butter two 9-inch round cake pans and line the bottoms with parchment paper.
Mix Dry Ingredients: In a bowl, whisk together flour, baking powder, baking soda, and salt.
Cream Wet Ingredients: In a large bowl or stand mixer, beat the sugar and eggs on medium speed for 2–3 minutes, until pale and slightly thickened. Add the vanilla, then add the oil with the mixer on low speed. Mix until fully combined.
Add Sourdough Discard: Reduce speed to low and mix in the discard.
Combine Batter: Add the dry ingredients in three parts, alternating with milk and ½ cup strawberry reduction. Mix until just combined. If desired, stir in a touch of natural or pink food coloring for a rosier color.
Bake: Divide batter evenly between pans. Smooth tops and bake for 28–32 minutes, until golden, springy, and a toothpick comes out clean.
Cool: Cool cakes in pans for 15 minutes. Remove from pans, peel off parchment, and cool completely on a wire rack. For easier decorating, freeze cakes for 30–60 minutes before frosting, be sure to wait until your cakes are fully cooled before popping into the freezer.