Prepare the Cake Pans: Start by heating your oven to 350°F. Grease two 9 inch round pans and line the bottoms with parchment paper. Lightly dust the sides with flour so the layers release cleanly after baking. I like to tap out the extra flour so the edges stay smooth.
Mix the Dry Ingredients: In a medium bowl, whisk the cake flour, cocoa powder, baking soda, and salt. Break up any cocoa lumps so the color stays even later. Set this bowl aside while you work on the batter.
Cream Butter and Sugar: In a large bowl, beat the softened butter with caster sugar until light and fluffy. This step adds air and helps the cake rise evenly. Take a moment here and scrape the bowl so everything blends smoothly.
Add Eggs and Oil: Beat in the eggs one at a time, mixing well after each. Pour in the vegetable oil and mix until the batter looks glossy and smooth. The oil keeps the cake extra moist, even after chilling.
Add Color and Flavor: Stir in the vanilla extract and red food coloring until the color looks deep and even. Add the vinegar and mix briefly. This step helps the baking soda activate and keeps the crumb tender.
Combine Wet and Dry: Add the dry ingredients in three parts, alternating with buttermilk. Start and end with dry ingredients. Mix gently until just combined. Stop as soon as the batter looks smooth to avoid a dense cake.
Bake the Layers: Divide the batter evenly between the pans. Smooth the tops and bake for 28 to 32 minutes until a toothpick comes out clean. Let the cakes cool in the pans for 10 minutes, then turn out onto a rack to cool fully.
