Preheat oven to 350 degrees F
Line 9x9 pan with with parchment so it hangs over the side so you can pull the cake out once cooled.
Measure out flour, put in small bowl set aside.
Measure out milk, put in small bowl, set aside.
In mixing bowl add butter, sugar, baking powder, salt and vanilla.
Mix on med/high until light and fluffy.
On medium speed add in one egg at a time, mix until well combined.
On low speed add the milk and flour, alternating until all add. Increase speed to medium.
Mix until well combined.
Pour batter into prepared pan.
Bake for 30 min or until golden brown and tooth pick inserted comes out clean.
In medium sauce pan add the egg whites and sugar.
On medium heat you are going to whisk egg mixture until sugar is completely dissolved (you can test it by rubbing a small amount between your fingers to test it, it should be completely smooth).
Transfer to mixing bowl and with whisk attachment whisk on high until stiff peaks form and mixture is cooled.
On low speed add the butter one piece at a time. Mix until well combined.
Switch to paddle attachment, add vanilla, mix until smooth and glossy.
Divide buttercream into 6 small bowls or cups.
Color each cup differently, I used pink, orange, yellow, green, blue, purple by Wilton.
Each cup of buttercream will be divided in half, plop half of each color randomly onto cake. Repeat with the second half of the buttercream.
Swirl the colors and spread the buttercream with a offset spatula or regular spatula.
Sprinkle with sugar crystals (optional).
When ready cut and serve.