Preheat the oven to 350°F (175°C). Grease three 8-inch cake pans, line the bottoms using parchment paper, then lightly coat the paper before dusting the interiors lightly using cake flour.

Whisk together cake flour, baking powder, baking soda, and salt inside a large bowl until evenly blended. Place this mixture aside while preparing the wet ingredients.

Beat softened butter and granulated sugar until the mixture becomes pale, fluffy, and noticeably larger in volume. Pour in vanilla extract, followed by egg whites a little at a time, mixing thoroughly after every addition.

Blend sour cream into the mixture until smooth. Alternate adding the dry mixture and whole milk, beginning and ending using the flour mixture. Stir only until everything comes together into a silky batter.

Divide the batter equally among the prepared pans. Bake until the tops become lightly golden and a toothpick inserted into the center comes out clean. Allow the cakes to rest briefly before transferring them onto a cooling rack.

Beat butter until creamy. Slowly add confectioners’ sugar, followed by heavy whipping cream. Continue mixing until the buttercream becomes airy. Blend in pink gel coloring until the frosting reaches a soft pastel shade.

Trim the cooled layers if necessary. Spread buttercream over the first layer, repeat between remaining layers, then cover the entire cake using an even coating. Smooth the surface carefully using an offset spatula.

Decorate the outside generously using rainbow sprinkles. Refrigerate briefly to help the frosting set before slicing. Serve at room temperature for the softest texture and the richest vanilla taste.