Preheat oven to 350 degrees. Grease and line four 6 inch pans. Set aside.
In a large bowl, sift together the flour, salt and baking powder. Set aside.
In the bowl of your electric mixer, mix the butter on high speed for about a minute and add the sugar all at once. Continue to mix on high speed until pale in colour and light and fluffy. About 5 minutes. Mix in the vanilla extract. Beat in eggs, one at a time and mixing well after each addition.
In three additions add the flour and the milk. Starting and ending with the flour. Gently stir in the flour with a spatula and be careful not to over mix. Stir in sprinkles and give a gentle stir. Evenly pour into prepared pans.
Bake around 20 minutes or until a toothpick inserted into the center comes our with just a few crumbs. Remove from oven and cool in pans for about 5 minutes on a wire rack. Remove cakes from pans and gently peel off parchment paper. Leave to cool completely.
While the cake is cooling, make the icing. In the bowl of your electric mixer, beat the butter on high speed until smooth. Gradually start adding your icing sugar and mixing as you go. Continue this process until you have added all of the icing sugar. It should be smooth and quite thick. Mix in the vanilla extract and two tablespoons of the milk. Mix on high speed. Add the remaining milk until smooth and creamy. If you think it is still too thick, add a little more milk until you reach the desired consistency.
To assemble the cake. Put your first layer flat side up on the cake plate or stand you are using. Spread over enough frosting to cover the entire layer. Repeat for remaining layers. For the top layer, place it with the flat side down, so you will have a mounded top. First frost the cake by almost scrapping the frosting against the cake to just cover it. This is the crumb layer, so crumbs won't show through in the final result. Then, frost the cake over completely until is it all covered. While the frosting is still really moist, throw over as many sprinkles as you like.
This cake can be stored in the fridge, but the frosting will harden. Make sure you take it out of the fridge at least an hour before cutting into it. It will keep up to 3 days but best the day it is made.