Prepare your pan and preheat: Set your oven to 160°C (140°C fan). Grease and flour a 23cm Bundt tin or a 23cm x 13cm loaf tin. The smell of the warming oven will be the first cue that baking is imminent.

Whisk the dry ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. The mixture should look pale and feel light and airy, with no lumps.

Cream the butter and sugar: In a large bowl, beat the butter and caster sugar with an electric mixer on medium-high speed for 4-5 minutes. The mixture will become pale, almost white, and fluffy; when you rub a bit between your fingers, you should feel no graininess from the sugar.

Add the eggs and vanilla: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. The batter will become smooth and glossy, and the vanilla aroma will fill your kitchen. Stir in the vanilla extract.

Alternate flour and buttermilk: With the mixer on low, add the flour mixture in three additions, alternating with the buttermilk in two additions. Begin and end with the flour. Mix only until just combined; you should see no streaks of flour, and the batter will be thick, silky, and pale.

Create the chocolate batter: In a small bowl, whisk the cocoa powder with the hot water and milk until you have a smooth, fudgy paste. It will smell intensely of dark chocolate. Fold one-third of the vanilla batter into the cocoa paste until completely combined. The chocolate batter will be darker and slightly thicker than the vanilla.

Layer and swirl: Spoon half of the remaining vanilla batter into the prepared tin, spreading it evenly. Spoon all of the chocolate batter on top, then finish with the remaining vanilla batter. Use a butter knife or skewer to create a figure-eight pattern through the batters, running from the bottom to the top. Do not over-swirl; you want distinct ribbons of colour, not a muddy grey.

Bake to perfection: Bake for 50-55 minutes. At the 45-minute mark, the kitchen will smell like a patisserie. The cake is ready when a skewer inserted into the centre comes out clean or with a few moist crumbs attached. The top should be a deep golden brown and spring back lightly when touched.

Cool with patience: Place the tin on a wire rack and let the cake cool in the tin for 15 minutes. The cake will release a gentle steam. After 15 minutes, invert it onto the rack and let it cool completely. The crust will be slightly crisp, and the interior will be tender.