1. Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
2. In a bowl, whisk together flour, baking soda, baking powder, and salt.
3. In another bowl, cream together sugar and butter until light and fluffy.
4. Add eggs one at a time, beating well after each addition.
5. Mix buttermilk and vanilla extract together.
6. Gradually add dry ingredients to the creamed mixture alternately with the buttermilk mixture, beginning and ending with dry ingredients. Mix until just combined.
7. Add purple food coloring (and cocoa powder if using) to achieve desired color.
8. Divide batter evenly between prepared pans.
9. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
10. Cool in pans for 10 minutes before removing to wire racks to cool completely.

For the Cream Cheese Frosting:

1. In a mixing bowl, beat cream cheese and butter until smooth.
2. Gradually add powdered sugar, 1 cup at a time, beating well after each addition.
3. Add vanilla extract and beat until light and fluffy.

Assembly:

1. Place one cake layer on a serving plate. Spread a layer of cream cheese frosting over the top.
2. Place the second cake layer on top and frost the top and sides of the cake with remaining frosting.
3. Optional: Decorate with purple sprinkles or edible glitter for extra flair.