Heat the oven to 180°C conventional or 160°C fan-forced. Grease and line a 20 cm square cake tin with baking paper, leaving a little overhang to help lift out the sponge.

Beat the softened butter, caster sugar and vanilla extract until pale and fluffy. Add the eggs one at a time, beating well after each addition; if the mixture begins to curdle, beat in a spoonful of the flour.

Fold in the self-raising flour alternately with the milk, mixing only until the batter is combined. Spread it evenly into the prepared tin so the cake bakes to an even thickness.

Bake for 20–25 minutes, until the top springs back when lightly pressed and a skewer inserted in the centre comes out clean. Cool in the tin for about 10 minutes, then turn the sponge onto a wire rack to cool completely.

Wrap the cooled cake and refrigerate it for at least 1 hour, or preferably overnight. Cut the chilled sponge into 16–20 squares, depending on the size you prefer.

For jam lamingtons, split each square horizontally or split the whole cake before cutting. Spread 150–200 g of raspberry or strawberry jam over the cut surfaces, then sandwich the sponge back together.

Combine the sifted icing sugar and cocoa powder in a bowl. Gradually whisk in the boiling water and melted butter until the icing is smooth and fairly fluid.

Dip each cake square into the chocolate icing, turning it carefully with forks to coat all sides. Let excess icing drain back into the bowl, then roll the square in desiccated coconut.

Place each finished lamington on a wire rack and allow the coating to set. Serve once the chocolate icing is firm, or refrigerate the cakes briefly if you prefer a cooler, denser finish.