Prepare Your Tins: Preheat your oven to 180°C (160°C fan). Grease and line the bases of two 20cm (8-inch) round cake tins with parchment paper.

Combine Dry Ingredients: In a medium bowl, whisk together the plain flour, cocoa powder, bicarbonate of soda, and salt. Sifting them together helps to aerate the mixture and prevent lumps.

Cream Butter and Sugar: In a large bowl or the bowl of a stand mixer, beat the room temperature butter and caster sugar together on medium-high speed for about 5 minutes, until the mixture is very light, pale, and fluffy. This step is vital for the cake's texture.

Add Eggs and Flavourings: Add the eggs one at a time, beating well after each addition and scraping down the sides of the bowl. Mix in the vanilla extract.

Mix the Blue Buttermilk: In a separate jug, whisk the buttermilk and the royal blue gel food colouring together until you have a smooth, evenly coloured liquid. I find starting with 1 teaspoon of gel and adding more until you reach your desired shade is the best approach.

Alternate Wet and Dry: With the mixer on low speed, add the dry ingredients to the butter mixture in three additions, alternating with the blue buttermilk in two additions. Begin and end with the dry ingredients. Mix only until just combined – over-mixing develops gluten and will make the cake tough.

Activate with Vinegar: In a small bowl, quickly stir the white vinegar into the batter. You might see it fizz slightly; this is the final acidic reaction that gives the cake its lift and tender crumb. Mix for just a few seconds to incorporate.

Bake the Cakes: Immediately divide the batter evenly between your prepared tins and smooth the tops. Bake for 30-35 minutes, or until a skewer inserted into the centre of the cakes comes out clean.

Cool Completely: Let the cakes cool in their tins for 10 minutes before carefully turning them out onto a wire rack to cool completely. It's crucial they are fully cool before you attempt to frost them, otherwise the frosting will melt.

Make the Frosting and Assemble: While the cakes cool, make the frosting. Beat the softened butter until smooth. Add the sifted icing sugar and beat on low speed until combined, then increase the speed to high and beat for 3-4 minutes until light and fluffy. Add the cold cream cheese and vanilla, and beat on medium-high for just 1-2 minutes until smooth and combined. Do not overbeat. Once the cakes are cool, place one layer on your serving plate, spread a generous layer of frosting on top, place the second cake layer on top, and use the remaining frosting to cover the top and sides of the cake.