In a bowl combine sifted flour, salt, baking soda and baking powder. Set aside.
In a separate bowl, beat together sugars and melted butter for 2 minutes, until sugars have dissolved. Add vanilla, egg yolks, sour cream and milk. Beat to combine.
Add dry ingredients and slowly mix them, until incorporated. Do not over mix.
Add sprinkles and stir with a spatula to combine.
Divide mixture between both pans. Bake for 35-50 minutes, until toothpick inserted comes out clean. You can cover with aluminum foil loosely at the 30 minute mark, to prevent the top from burning.
Remove from the oven, let the cakes cool for 10 minutes in the pans and transfer to a cooling rack. Cool completely.
When ready to assemble, cut off the domes (if any) and using a serrated knife cut the cake layers in half, through the middle.
Beat butter on medium-high for 1 minute. Slowly add sifter powdered sugar. Beat on medium-high for 2 minutes. Scrape down the sides and bottom of the bowl. Add vanilla. Beat for 2 more minutes.
Add milk to and beat to combine. If the frosting is too stiff, add more milk to reach the desired consistency.
Spread about 1/3 cup of buttercream between each layer. Since I have 4 layers, I used the darkest buttercream between layers 1 and 2, then the medium shade between 2 and 3 and the lightest between 3 and 4. I used the lightest shade of buttercream to crumb coat the cake and then used some more on top of it. Refrigerate for 15 minutes.
Pipe 2 large dots of icing one over another, using the darkest shade of buttercream. Then pipe one from the medium shade on top and another one from the lightest shade over it, make sure the tots are covering the side of the cake completely. Using an offset spatula, smear the dots to flatten half of them, making them look like petals. Repeat all over the cake. Refrigerate for at least 30 minutes.