01 - Preheat oven to 350°F. Grease an 8-inch square baking pan and line with parchment paper, leaving overhang on two sides for easy removal.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until evenly distributed. Set aside.
03 - In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium-high speed until pale, light, and fluffy, approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating thoroughly after each addition until fully incorporated. Mix in the vanilla extract.
05 - Gradually add the flour mixture to the butter mixture in three additions, alternating with the milk in two additions, mixing on low speed after each until just combined. Do not overmix.
06 - Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 20 to 25 minutes until a toothpick inserted into the center comes out clean and the top springs back when lightly pressed.
07 - Allow the cake to cool completely in the pan, then turn out onto a wire rack. Wrap tightly in plastic wrap and freeze for 30 minutes to firm the crumb for easier dipping and handling.
08 - Combine the raspberries, water, granulated sugar, and lemon juice in a medium saucepan. Bring to a simmer over medium heat and cook for 5 to 7 minutes, stirring occasionally, until the berries break down and the mixture thickens slightly.
09 - Pass the syrup through a fine mesh sieve into a bowl, pressing on the solids to extract maximum flavor. Discard the seeds and allow the syrup to cool to room temperature.
10 - Using a serrated knife, trim the edges of the sponge if desired for uniformity, then cut into 12 even squares.
11 - Spread the desiccated coconut in a wide, shallow bowl. Dip each sponge square briefly into the cooled raspberry syrup, allowing excess to drip off, then roll gently in the coconut until evenly coated on all sides.
12 - For filled lamingtons, slice each coated square in half horizontally. Spread the bottom half with raspberry jam and a dollop of whipped cream, then sandwich with the top half.
13 - Place the finished lamingtons on a wire rack and let them set for 15 minutes at room temperature before serving.