150 g self-raising flour (1 1/4 cups), or all-purpose with 1 1/2 tsp baking powder
30 g cornstarch (1/4 cup)
60 g melted butter (4 tbsp)
60 ml warm milk (1/4 cup)
Icing: 400 g powdered sugar (3 1/4 cups)
40 g cocoa powder (1/3 cup + 1 tbsp)
20 g butter (1 1/2 tbsp)
150 ml boiling water (2/3 cup)
250 g desiccated coconut (2 3/4 cups)
Optional: raspberry jam and whipped cream for filling
How to make it
Sponge (15 min): Heat the oven to 180C (350F) and line a 20 cm (8 in) square tin. Whisk eggs, sugar and vanilla on high for 8 to 10 minutes, until tripled and thick enough to leave a trail.
Fold (5 min): Sift the flour and cornstarch over and fold in gently in three goes. Fold in the butter and milk mixed together, without knocking out the air.
Bake (25 min): Bake 25 minutes until springy and pulling from the sides. Cool, then wrap and leave overnight (or freeze 1 hour) so it cuts without crumbling.
Icing (5 min): Whisk powdered sugar, cocoa, butter and boiling water in a heatproof bowl until smooth and runny. Keep it warm over a pan of hot water.
Dip (20 min): Trim the cake and cut into 16 squares. Using two forks, dip each square in the icing, let the excess drip, then roll in coconut. Set on a rack for 1 hour.
Fill (optional, 10 min): Split each one, spread with jam and whipped cream and sandwich back together.
Tips
Thin the icing with a splash of hot water if it thickens; thick icing tears the sponge.
Tokyo Lamington-style: swap the chocolate for a lemon or yuzu icing and pipe toasted meringue on top.