Filipino Mocha Chiffon Cake

Method

1. Heat the oven to 165°C and leave a clean 25 cm removable-bottom tube pan ungreased. Dissolve the coffee and cocoa in the hot water; cool to lukewarm.
2. Sift the cake flour, baking powder, and salt twice. Whisk the yolks with 150 g sugar until thickened, then whisk in the coffee mixture, oil, and vanilla.
3. Whisk in the dry ingredients just until smooth, scraping the bowl so no flour pockets remain. Stop before the batter becomes elastic.
4. Beat the whites until foamy, add cream of tartar, then gradually beat in 100 g sugar to glossy medium-stiff peaks. Stop before they look dry or clumpy.
5. Fold one-third of the whites into the batter. Fold in the remainder in two additions, reaching the bowl's bottom, until no white streaks remain.
6. Pour into the pan and run a skewer through the batter to release large bubbles. Bake 45–50 minutes without opening for 35 minutes, until the top springs back and a skewer is clean.
7. Immediately invert the pan on its feet or a sturdy bottle and cool for at least 90 minutes. When cold, loosen the sides, tube, and base with a thin knife.
8. Beat the butter for 3 minutes. Beat in icing sugar, cocoa, and salt in three additions, followed by the cooled coffee and 2 tablespoons milk. Add more milk if stiff; chill 10 minutes if soft.
9. Frost the completely cooled cake and leave the centre open. Let the buttercream set for 20 minutes, then slice with a serrated knife.