Preheat oven to 175°C / 350°F (non fan-assisted). Grease an 8x4-inch loaf tin and line the bottom with parchment paper. Set aside.
Place egg whites in a clean, dry, and oil-free metal bowl. Using an electric mixer, beat the egg whites on medium speed until foamy. Add in Part A of the sugar gradually, while beating the egg whites constantly. After the sugar has been added, turn the speed up to high and beat until the egg whites form stiff peaks. Set aside.
Place butter and Part B sugar in a separate mixing bowl. Without cleaning the beaters, cream the butter and sugar on medium-high speed until light and fluffy, about 5 minutes. Beat in the egg yolks, one by one, until incorporated before adding the next egg yolk. Beat in the vanilla with the last egg yolk.
Sift half of the flour into the butter mixture, then beat on low until just incorporated. With the mixer on low, beat in the milk. Sift in the rest of the flour and bit on low until just incorporated.
Using a big rubber spatula, fold in the egg whites in 3 parts (fold in ⅓ first, then another ⅓, then remaining egg whites). Make sure to scrape the bottom of the mixing bowl to ensure the batter is well-mixed.
Scrape the batter into the loaf tin and smooth the top. Bake for 30 minutes, then lower the temperature to 170°C / 338°F and continue baking for 10 to 15 minutes, until a skewer inserted to the center of the cake comes out clean and the cake is pulling away from the edges of the loaf tin.
Transfer the loaf tin on a wire rack and cool the cake for 30 minutes. Run a thin spatula along the edges of the cake, turn the cake out, peel off the parchment paper and turn the cake right side up onto a rack and let cool completely.
Store any leftover cake in an airtight container at room temperature for up to 3 days. The cake can also be stored in the refrigerator for up to 5 days.
Make sure you have all the ingredients ready before you start making the cake. The whipped egg whites should not be set aside for too long as it will become watery, so you must cream the butter immediately after you finish whipping the egg whites. If you have 2 mixers, you can cream the butter and whip the egg whites at the same time.
Use only good-quality butter for this recipe! In Singapore I would have used SCS but as I'm overseas, I used Lurpak butter.