01 -  Preheat oven to 350°F. Grease and line a 9x13-inch sheet pan with parchment paper for easy removal. 
 02 -  Whisk flour, granulated sugar, baking powder, and salt together in a large mixing bowl. 
 03 -  In another bowl, whisk whole milk, melted butter, eggs, vanilla extract, and sour cream until fully integrated and smooth. 
 04 -  Gently pour the wet mixture into the dry ingredients, stirring until just combined without overmixing. 
 05 -  Transfer batter to the prepared pan and evenly spread with a spatula. 
 06 -  Bake on the center rack for 28–32 minutes or until a toothpick inserted into the center yields clean results. 
 07 -  Allow the cake to cool completely in the pan set on a wire rack before frosting. 
 08 -  Beat softened butter in a bowl until creamy. Gradually incorporate sifted powdered sugar, then add milk, vanilla extract, and a pinch of salt. Beat until light and fluffy. Divide and tint with food coloring if desired. 
 09 -  Swirl or dollop frosting over the cooled cake, embracing natural imperfection and texture. 
 10 -  Generously scatter rainbow sprinkles over the frosted surface. 
 11 -  Slice directly in the pan and present servings as desired.