Preheat oven to 325 degrees. Grease loaf pan with nonstick spray and dust with cocoa powder. Tap out the excess powder. Set aside. Add flour, cocoa powder, salt, and baking soda to a medium bowl. Whisk together. Set aside.
In a bowl fitted for a stand mixer, add softened butter and granulated sugar. Beat together until fluffy. While mixer is on low, add eggs one at a time, scrapping down the sides of the bowl after each one. Add vanilla extract and vinegar and combine.
Add flour mixture in three batches, alternating with the buttermilk, making sure to start and end with the flour mixture. Add food coloring and mix just until combined. Do not overmix!
Pour cake mix into prepared loaf pan. Smooth the top and place into oven to bake for 1 hour to 1 hour 15 minutes. Make sure to start checking the cake around 1 hour to see if it's done by sticking a toothpick in the center. When it comes out clean, it's ready. You don't want to overbake the cake or it will be dry.
Once the cake is done, remove from the oven. Let it cool in the pan for 15 minutes. Then, use a knife to release the cake from the edges of the pan. Remove the cake from the pan and let it finish cooling. In the meantime, make the frosting.