Preheat the oven to 350°F. Prepare three 8-inch round cake pans by lining the bottoms with circles of parchment paper and spraying the sides with baking spray.
In a large bowl, cream the butter and sugar and beat on medium speed until light and fluffy, about 2-3 minutes.
1 1/2 cups (300g) granulated sugar, 3/4 cup (170g) salted butter
Add the egg, egg whites, and vanilla. Mix well, stopping to scrape the bottom and sides of the bowl.
1 large egg, 4 large egg whites, 1 teaspoon pure vanilla extract
Add half of the flour and baking powder, mixing on low speed just until combined with a few streaks of flour remaining.
2 1/2 cups (353g) all-purpose flour, 4 teaspoons baking powder
Add the sour cream and maraschino cherry juice. Mix well to combine.
3/4 cup full-fat sour cream, 1/2 cup maraschino cherry juice
Add the remaining flour, mixing just until combined. Add the milk and mix again, stopping to scrape the bottom and sides of the bowl between additions. Do not overmix.
1/4 cup whole milk
Stir in the chopped maraschino cherries.
3/4 cup chopped maraschino cherries
Divide the batter evenly between the prepared cake pans, then bake for 24 to 26 minutes or until a cake tester inserted in the center of the cakes comes out with only a few crumbs.
Cool completely on a wire rack.