1. Prepare two 8- or 9-inch cake pans by spraying them with cooking spray. Cut a circle of parchment paper to line the bottom of each pan, and spray the parchment with cooking spray. Set aside.
2. Preheat oven to 350 degrees F.
3. In a large bowl, whisk together the flours, baking soda, and salt. Set aside.
4. In a medium bowl, combine the buttermilk, food coloring, vinegar, and vanilla. Set aside.
5. In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter and dark brown sugar until light and fluffy, about 3-4 minutes. Add eggs and continue beating on medium speed for another minute until thoroughly combined.
6. Add flour mixture and buttermilk mixture in alternating additions, beginning and ending with the flour mixture, scraping bowl as needed.
7. Divide batter evenly between the prepared cake pans.
8. Bake on middle oven rack for 35-45 minutes, or until the top of the cake springs back when pressed gently, or a toothpick comes out clean. (8-inch pans will take a few minutes longer than 9-inch pans.)
9. Cool in pans for 10 minutes, then remove to a wire rack to cool completely before frosting.

To Make The Frosting-

1. In the bowl of a stand mixer, beat the butter and cream cheese together until smooth. Add vanilla and salt and beat until combined. Slowly add the confectioners sugar, and beat until very smooth, light and fluffy, about 5 minutes.
2. Refrigerate for 15 minutes before frosting the cake.