Preheat your oven to 350 degrees F (180 degrees C) . Line a 9-inch spring form pan or cake pan with parchment paper. In a bowl mix together the All-Purpose Flour (3 cup),Salt (1 teaspoon), Granulated Sugar (1 1/4 cup), Baking Soda (1/2 teaspoon), Baking Powder (2 teaspoon), Brown Sugar (1/2 cup).
In another bowl or a jug, whisk the Milk (1 cup), Egg (2), Sour Cream (2/3 cup),and Pure Vanilla Extract (2 tablespoon) until combined.
Pour the wet ingredients into the dry, and gently stir. Add in the Butter (1/2 cup). Do not panic if the batter is thick, that is how it should be. DON'T OVER MIX, it will make your cake tough.
Divide the cake into 6 bowls. Add enough drops of Blue Food Coloring (to taste), Red Food Coloring (to taste), Yellow Food Coloring (to taste), Orange Food Coloring (to taste), Green Food Coloring (to taste), Purple Food Coloring (to taste) to each bowl. Mix until you have reached your desired color.
Spoon the batter into your prepared pan, scatter each color on top of each other. Bake for 70 minutes or until a tooth pick inserted to the middle comes out clean.
Beat the Butter (1/2 cup) on it's own until is nice and smooth. add in the Powdered Confectioners Sugar (4 1/4 cup) and Pure Vanilla Extract (1 tablespoon) extract and beat on medium speed for 4 minutes.
Once your cake is out of the oven let it cool COMPLETELY. Slice some layers. Spread the frosting in the layers. Then spread some frosting on top and around the cake.