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