1. Preheat oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.2. In a large bowl, whisk together the dry ingredients: flour, sugar, baking soda, salt, and cocoa powder.3. In a medium bowl, whisk together the wet ingredients: melted butter, eggs, buttermilk, purple food coloring, and vanilla extract.4. Pour the wet ingredients into the dry ingredients and whisk by hand until just combined. Do not overmix the batter.5. Pour the batter into the prepared 9×13 pan and spread into an even layer.6. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.7. Let the cake cool completely in the pan on a wire rack (at least 1 hour).8. To make the frosting: In a large bowl, beat the softened cream cheese and butter with an electric mixer on medium-high speed until smooth and fluffy (2-3 minutes).9. Add the powdered sugar and vanilla extract. Mix on low speed until combined, then increase to medium-high and beat for 2 minutes until light and airy.10. Spread the frosting evenly over the cooled cake directly in the pan.