Preheat the oven to 350°F. Spray a 9x13 baking dish with baking spray, set aside. 

In a medium-sized bowl, mix together the flour, baking powder, salt, and baking soda, set aside.

In the body of a stand mixer with the paddle attachment, cream the butter with the sugar for 3 minutes until light and fluffy, scrape down the sides as needed.

Add the egg whites into the mixer in three batches, mixing them in fully before the next addition, scrape down the sides.

Slowly stir in the milk and vanilla until smooth.

Add half of the flour mixture and mix in until combined.

Mix in half of the sour cream until combined.

Repeat with the remaining flour and sour cream.

Scrape down the sides and stir in the sprinkles.

Pour the batter into the baking dish and smooth out the top.

Bake for 35-42 minutes until a toothpick inserted into the center comes out clean.

Place on a wire rack to cool completely.

While the cake is cooling, make the frosting by placing the butter into a large bowl with an electric hand mixer and whip until smooth.

Slowly add the powdered sugar 1 cup at a time until incorporated.

Add the salt, milk, and vanilla, stir it in until combined then whip it on medium-high speed until light and fluffy for 3 minutes.

Frost the cake and add more sprinkles if desired, cut and serve.