Preheat the oven to 350°F (180°C) and line a rectangular cake pan with parchment paper.
Sift together the baking soda, cornstarch, custard powder, and cream of tartar three times to ensure an even texture.
In a large mixing bowl, beat the egg whites until soft peaks form. Gradually add the sugar, one tablespoon at a time, and continue beating until stiff peaks form.
Add the egg yolks one at a time, beating well after each addition.
Gently fold in the sifted dry ingredients in two batches, ensuring the batter stays light and airy.
Pour the batter into the prepared cake pan and bake for 15-20 minutes, or until golden and springy to the touch.
Allow the cake to cool completely on a wire rack before cutting it into squares.
Dissolve the raspberry jelly in 1 cup of boiling water, stirring until fully combined. Let it cool slightly until it thickens.
Dip each cake square into the raspberry jelly, ensuring an even coating, then roll in desiccated coconut.
Once set, slice each lamington in half and spread a layer of whipped cream before sandwiching it back together.