Preheat the oven to 350°F (175°C) and grease two 9-inch round cake pans.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a large bowl, beat the butter and sugar until fluffy. Add eggs one at a time, mixing well after each.
Mix in the buttermilk, vanilla extract, and purple food coloring until the batter is evenly colored.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Divide the batter evenly between the prepared pans and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
Frost and decorate as desired.