Preheat and Prepare: First, preheat your oven to 180°C (160°C Fan / Gas Mark 4). Grease and line a 23cm (9-inch) square baking tin with parchment paper, leaving an overhang on two sides to help you lift the cake out later.

Combine Dry Ingredients: In a medium bowl, whisk together the plain flour, baking powder, bicarbonate of soda, and salt. Set this aside.

Cream Butter and Sugar: In a large bowl, using a stand mixer or an electric hand whisk, beat the softened butter and caster sugar together on medium-high speed for about 3-4 minutes. You're looking for a light, pale, and fluffy texture. This step incorporates air, which is vital for a tender cake. You can learn more about the science of creaming butter and sugar to perfect your technique.

Add Wet Ingredients: Add the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl, then beat in the vanilla extract.

Alternate Flour and Buttermilk: Reduce the mixer speed to low. Add the flour mixture in three additions, alternating with the buttermilk in two additions, beginning and ending with the flour. Mix only until just combined – be careful not to overmix at this stage.

Make the Cinnamon Swirl: In a separate small bowl, mix together the melted butter, dark brown sugar, and ground cinnamon until it forms a thick, sandy paste.

Layer and Swirl: Pour half of the cake batter into your prepared tin and spread it evenly. Dollop half of the cinnamon mixture over the batter. I find that using two teaspoons to drop small spoonfuls works best for me. Pour the remaining cake batter on top and smooth it out. Dollop the rest of the cinnamon mixture over the top.

Create the Marble Effect: Take a butter knife or a skewer and gently drag it through the batter in a figure-of-eight motion 3-4 times to create a marbled effect. Don't overdo it, or you'll lose the distinct swirl.

Bake the Cake: Bake for 30-35 minutes, or until the cake is golden brown and a skewer inserted into the centre comes out clean with a few moist crumbs attached. Let the cake cool in the tin on a wire rack for 15 minutes.

Prepare the Icing and Finish: While the cake is cooling, make the icing. In a medium bowl, beat the softened cream cheese and butter until smooth. Gradually beat in the sifted icing sugar until fully combined, then mix in the vanilla. If the icing is too thick, add a tablespoon of milk. Spread the icing over the still-warm cake and serve.