Preheat oven to 180C. Butter two 23cm-diameter cake tins, then dust with flour and line bases with baking paper. Whisk eggwhite, 120ml coconut milk, coconut essence and vanilla in a bowl and set aside. Beat butter and remaining coconut milk in an electric mixer until combined, then add flour, sugar, baking powder, desiccated coconut and a pinch of salt and mix on low until combined. Beat on high for another minute, scraping down sides of bowl, and add eggwhite mixture in 3 batches. Divide between tins and bake until golden and a skewer inserted comes out clean (35-40 minutes). Cool in tins for 15 minutes, then turn out onto wire racks and cool completely.