Preheat the oven to 175 C. Grease a 9" round cake tin. Set aside.
Sift the flour and baking powder in a large bowl. Mix in the salt if using, then set aside.
Beat the butter and sugar in a mixing bowl till pale and fluffy. Add the eggs and vanilla beat well till combined.
Fold in the flour mixture, and beat until smooth and combined. The batter is fairly thick, but if it's too thick to spread, thin it out with a splash of milk. Gently fold in the sprinkles making sure they're evenly dispersed. Spoon the batter into the prepared tin and smooth it out.
Bake for 20 mins, then check to see if the top is browning too fast. If so, cover with aluminium foil and continue to bake for about 10 to 15 mins till a toothpick poked in the center comes out clean. Rotate the tin halfway through the baking time for even browning. If you have two 9" pans and your oven can accommodate both, split the batter in half and bake two single layers which bake quite flat. Baking time will be about 5 minutes less per layer.
Let the cake cool completely at room temperature.
To make the frosting, beat the cream cheese and butter together till smooth. Add the sugar and vanilla and beat again till well combined. Taste and add more sugar if you prefer. Add the food colour if using and beat till uniformly coloured. I kept 1/4 of the frosting aside for the little white blobs on top and coloured the remaining frosting pink. Chill the frosting for about 15 minutes so it's easier to spread, but doesn't harden.
Slice the cake into two even layers horizontally. If the top has domed, even that out first.
Spread about half the frosting on the bottom layer, making sure it's even. Place the top half on it and press down gently. Now cover the top and sides with remaining frosting. If you'd like to pipe any designs on top, first chill the cake for 10 minutes, then proceed. Decorate with sprinkles!
Slice and enjoy! Store the cake in the fridge at all times, but keep at room temperature for about 30 minutes before eating before butter cakes harden once refrigerated. Happy baking!