Prepare the Loaf Pan: Preheat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving some parchment hanging over the sides for easy removal.
Mix the Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt until evenly combined. Set aside.
Cream the Butter and Sugar: In a large mixing bowl, beat the softened butter and granulated sugar until light, fluffy, and creamy, about 2–3 minutes.
Add the Eggs: Beat in the eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl if needed.
Add the Wet Ingredients: Mix in the vegetable oil, buttermilk, vanilla extract, red food coloring, and white vinegar until the mixture is smooth and evenly colored.
Combine the Wet and Dry Mixtures: Gradually add the dry ingredients to the wet mixture. Mix gently just until the flour disappears. Avoid overmixing, as this can make the loaf dense.
Fill the Loaf Pan: Pour the red velvet batter into the prepared loaf pan. Smooth the surface with a spatula and gently tap the pan on the counter to release any large air bubbles.
Bake the Cake: Place the loaf in the preheated oven and bake for 45–55 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool Completely: Let the cake cool in the pan for 10–15 minutes. Carefully lift it out using the parchment paper and transfer it to a wire rack. Allow it to cool completely before frosting.
Make the Vanilla Frosting: Beat the softened butter until creamy. Gradually add the powdered sugar and mix until smooth. Add the vanilla extract, salt, and heavy cream. Beat until the frosting is light, fluffy, and easy to spread.
Frost the Loaf: Spread the vanilla frosting evenly over the completely cooled red velvet loaf. Use an offset spatula or the back of a spoon to create soft decorative swirls.
Decorate and Serve: If desired, sprinkle red velvet cake crumbs over the frosting for decoration. Slice the loaf with a sharp knife and serve.