01 - Preheat oven to 350°F. Grease an 8-inch square baking pan and line with parchment paper, leaving an 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 - Using an electric mixer, beat the softened butter and granulated sugar in a large bowl 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 incorporated. Scrape down the sides of the bowl as needed. Mix in the vanilla extract.
05 - Gradually add the dry ingredient mixture to the butter mixture in three additions, alternating with the whole milk in two additions. Fold gently until just combined, being careful not to overmix.
06 - Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 20–25 minutes until a toothpick inserted into the center emerges clean. Allow the cake to 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 up the crumb, which makes dipping and coating significantly easier.
08 - Combine the raspberries, water, granulated sugar, and lemon juice in a medium saucepan. Simmer over medium heat for 5–7 minutes, stirring occasionally, until the sugar dissolves and the raspberries break down. Strain through a fine mesh sieve to remove the seeds and let the syrup cool to room temperature.
09 - Using a serrated knife, trim the edges of the sponge cake if desired for a neater appearance, then cut into 12 even squares.
10 - Spread the desiccated coconut in a wide, shallow bowl. Working one at a time, dip each cake square briefly into the cooled raspberry syrup, allowing excess to drip off. Roll the dipped square in the coconut, pressing gently to coat all sides evenly.
11 - For filled lamingtons, slice each coated square in half horizontally. Spread the bottom half with raspberry jam and pipe or spoon whipped cream on top. Sandwich the halves together before the final coconut coating step.
12 - Place the finished lamingtons on a wire rack and allow them to set for 15 minutes at room temperature before serving.