Preheat oven to 160°C. Measure and prepare the ingredients as detailed above. Line bottom of 18 x 6 cm round cake pan with 2 layers of parchment paper.

Cook butter over medium-low heat, stirring with whisk, till just bubbling thoroughly. Turn off heat. (If your stove is electric, remove pan from stove.) Add cake flour. Stir till smooth. Transfer into mixing bowl with spatula.

Add milk. Mix vigorously with whisk till just combined. Feel sides of bowl, which should be very hot. Wait till edge of heat eases, about 30 seconds.

Add vanilla extract, salt and 20 g egg white. Whisk till combined. Add egg yolks in 3 batches. Whisk till combined after each addition.

Whisk 105 g egg whites on medium-low speed till frothy. Add cream of tartar. Whisk till thick foam forms. Gradually add castor sugar, still whisking. Continue to whisk till soft peak stage. Reduce speed to low. Whisk till firm peak stage (i.e. peak is hooked).

Yolk mixture is now slightly bubbly. Stir with whisk to remove bubbles. Add egg whites in 3 batches. Mix with whisk till almost even after each addition. Scrape down and fold with spatula till just even, banging mixing bowl against worktop 2-3 times.

Transfer 20 g batter into small bowl. Add 1 tsp cocoa powder. Mix thoroughly. Pour remaining batter into cake pan in 3 batches, slowly and from 30 cm high. After each batch, dot with cocoa batter and drag sharp end of skewer through dots. Jiggle batter till level, tapping pan against worktop 3-4 times.

Bake cake in bottom of oven till batter doesn't jiggle when shaken, 22-23 minutes. Move pan to lower-middle shelf. Increase temperature to 180°C. Bake till middle of cake doesn't make squishing sound when pressed, about 10-12 minutes.

Remove cake from oven. Leave on wire rack, inverted, till cool. Knock sides of pan till cake is loosened. Invert and remove pan. Invert cake right way up. Cut and serve.