Preheat the oven to 170°C (340°F). Grease and line three 6-inch round cake tins with parchment paper.
Combine the dry ingredients. In a medium bowl, sift together the flour, cocoa powder, baking soda, and salt. Set aside.
Cream the butter and sugar. In a large bowl or stand mixer, beat the butter and sugar together for 2–3 minutes until light and fluffy.
Add the eggs and vanilla. Beat in the eggs one at a time, then mix in the vanilla extract. Scrape down the sides of the bowl as needed.
Mix the wet ingredients. In a jug, whisk together the buttermilk, vinegar, and red food colouring.
Combine the mixtures. Add the dry ingredients to the butter mixture in three additions, alternating with the buttermilk mixture.
Start and end with the dry ingredients. Mix until just combined - don’t overmix.
Divide and bake. Evenly divide the batter between the three tins. Bake for 30–35 minutes or until a skewer inserted in the centre comes out clean.
Let the cakes cool in the tins for 10 minutes, then transfer to a wire rack to cool completely before decorating.
Make the buttercream by combining the butter, cream cheese, vanilla and icing sugar. Pipe onto the fully cooled cakes.