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 emulsified. Stir in the vanilla extract.
05 - Gradually add the dry flour mixture to the butter mixture in three additions, alternating with the whole milk in two additions, beginning and ending with the flour. Fold gently until just combined; avoid overmixing.
06 - Pour the batter into the prepared pan and smooth the surface with a spatula. Bake for 20 to 25 minutes until the top is golden and a toothpick inserted into the center emerges clean.
07 - Let the cake cool completely in the pan set on a wire rack. Turn the sponge out onto the rack. For cleaner dipping, wrap the cooled sponge in plastic and freeze for 30 minutes to firm the crumb.
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 sugar dissolves. Strain through a fine mesh sieve to remove seeds and let the syrup cool to room temperature.
09 - Using a serrated knife, trim the edges of the sponge if desired, then cut into 12 even squares approximately 2 inches each.
10 - 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.
11 - For filled lamingtons, slice each coated square horizontally through the middle. Spread the bottom half with raspberry jam and pipe or spread whipped cream on top. Sandwich the halves together before or after coating with coconut.
12 - Place the finished lamingtons on a wire rack and allow them to set for 15 minutes at room temperature before serving.