Heat 5cm water in a large steamer or, if you do not have one large enough to hold a 23cm cake tin, a large pan with an upturned tin or metal plate in the base. Grease a 23cm round cake tin (not a loose-based one), about 7.5cm deep.
Sift the flour and baking powder together. Put the egg whites in a very large bowl with a third of the sugar. Using an electric mixer, whisk on high speed until they start to form peaks, then add another third of the sugar and continue to whisk on high. Once stiff peaks have formed, add the remaining sugar, plus the salt and lemon juice, and whisk for a further minute, until firm. Fold in the flour with a large metal spoon, taking care not to lose the volume. Fold in the oil and vanilla. The mixture should be a thick batter; if it seems a little too thick, add the milk.
Transfer half the mixture to another bowl. Mix in the cocoa powder and black food colouring, then transfer the mixture to a jug. Add the strawberry flavouring and strawberry apple pieces to the remaining mixture and transfer to a second jug.
Pour the brown batter into the centre of the prepared cake tin, creating a circle about 10cm in diameter; it will spread out to the sides as you add more batter. Next pour the white batter into the centre of the brown batter, in a circle about 7.5cm in diameter. Keep alternating between white and brown batter until they are both used up. Cover the top of the tin with lightly greased greaseproof paper and secure with string. Wrap the top in foil, tucking it under the lip of the tin.
Place in the steamer, or in the pan of water on the upturned tin or plate. Steam for 2 1/2-3 hours, topping up the water as necessary, until the cake is firm to the touch and a skewer inserted in the centre comes out clean (you will have to unwrap it to check). Lift the cake out of the steamer using oven gloves. Leave for 10 minutes, then remove from the tin and leave to cool. Finish with a dusting of icing sugar and sprinkle over a few strawberry-apple pieces.