01 - Preheat the oven to 350°F. Grease an 8-inch square baking pan and line it with parchment paper.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat the softened butter and granulated sugar with an electric mixer until light and fluffy. Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.
04 - Gradually add the flour mixture to the butter mixture, alternating with the whole milk, mixing until just combined. Do not overmix.
05 - Pour the batter into the prepared pan and smooth the top evenly. Bake for 20 to 25 minutes, or until a toothpick inserted in the center comes out clean.
06 - Allow the cake to cool completely in the pan, then turn it out onto a wire rack. For easier handling during dipping, wrap the sponge and freeze for 30 minutes.
07 - Combine the raspberries, water, granulated sugar, and lemon juice in a saucepan. Simmer over medium heat for 5 to 7 minutes, stirring occasionally. Strain through a fine mesh sieve to remove the seeds and let the syrup cool.
08 - Using a serrated knife, cut the cooled sponge cake into 12 even squares.
09 - If adding filling, cut each sponge square in half horizontally. Spread the bottom half with raspberry jam and a dollop of whipped cream, then sandwich with the top half.
10 - Place 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 in the coconut to coat all sides evenly.
11 - Place the finished lamingtons on a wire rack and let them set for 15 minutes before serving.