01 - Preheat oven to 350°F. Grease an 8-inch square cake pan and line the bottom with parchment paper.
02 - Sift flour, baking powder, and salt into a medium bowl. Set aside.
03 - In a large bowl, beat softened butter and granulated sugar together using an electric mixer until pale, light, and fluffy, approximately 3 to 4 minutes.
04 - Add eggs one at a time, beating well after each addition. Stir in vanilla extract until fully incorporated.
05 - Gradually add the sifted dry ingredients to the wet mixture in two additions, alternating with milk. Fold gently until just combined, being careful not to overmix.
06 - Pour batter into the prepared pan and smooth the top with a spatula. Bake for 22 to 25 minutes, or until a toothpick inserted into the center comes out clean.
07 - Allow the cake to cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.
08 - Once completely cool, trim the edges if desired and cut the cake into 12 equal squares. Place squares in the freezer for 30 minutes to firm up, which makes coating easier.
09 - Warm raspberry jam with hot water in a saucepan over low heat, stirring until smooth and pourable. Transfer to a shallow bowl.
10 - Spread desiccated coconut evenly in a separate shallow bowl or plate. Set up a wire rack over a baking sheet for placing finished lamingtons.
11 - Using two forks, dip each frozen cake square into the warm raspberry mixture, turning to coat all sides evenly. Allow excess to drip off, then roll gently in desiccated coconut until fully covered. Place on the wire rack.
12 - Let the coated lamingtons set at room temperature for at least 30 minutes before serving to allow the coating to firm up.