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 minutes.
04 - Add the eggs one at a time, beating thoroughly after each addition until fully emulsified. Stir in the vanilla extract.
05 - Gradually add the flour mixture to the butter mixture in three additions, alternating with the whole milk in two additions, mixing on low speed 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. Let cool completely in the pan.
07 - Turn the cooled sponge out onto a wire rack. Wrap tightly in plastic wrap and freeze for 30 minutes to firm the crumb, making it easier to handle during dipping.
08 - Combine raspberries, water, granulated sugar, and lemon juice in a saucepan. Simmer over medium heat for 5 to 7 minutes, stirring occasionally, until the raspberries break down. Strain through a fine mesh sieve to remove seeds and let cool to room temperature.
09 - Using a serrated knife, cut the chilled sponge cake into 12 even squares.
10 - Spread 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.
11 - For filled lamingtons, slice each sponge square in half horizontally. Spread the bottom half with raspberry jam, pipe or spread whipped cream on top, and sandwich with the upper half before dipping in syrup and coating with coconut.
12 - Place finished lamingtons on a wire rack and allow to set for 15 minutes at room temperature before serving.