Butter cake with beautiful chocolate swirls made using the egg separation method - which makes it light and tender yet still buttery and delicious! Double the recipe to make in an 8-inch tin.
Preheat oven to 160°C (conventional) / 150°C (convectional). Line a 8x4-inch baking tin with parchment paper.
(A) Eggs: Separate egg whites and egg yolks. Place egg whites in a clean, dry and grease-free metal bowl. Whisk egg yolks with milk.
2 eggs, 30 grams milk
(B) Dry Ingredients: Sift plain flour, cornstarch and baking powder. Use a wire whisk and whisk the mixture 50 strokes to evenly distribute the baking powder. Set aside.
(C) Cocoa Paste: Whisk cocoa powder and boiling water together until no lumps remain. Set aside.
10 grams cocoa powder, 15 grams boiling water
(D) Egg White Meringue: Get ready an electric mixer. Add sugar into egg whites and whip on medium-high spped until meringue forms firm peaks - the peaks will hold but the tip will bend over.
25 grams caster sugar
(E) Vanilla Batter: Weigh the mixing bowl that you are going to use to make the vanilla batter and note down the weight. Place butter, sugar, salt and vanilla in another mixing bowl. Without cleaning the beaters, cream the butter on medium-high speed until pale, light and fluffy, about 5 minutes, scrapping down the sides every 2 minutes.
Add in egg yolk-milk mixture in two batches, beating on medium speed until fully incorporated and scrapping down the sides before the next addition.
Add in the flour mixture in two batches, beating on low speed until just incorporated. Scrape the sides and bottom of the bowl to ensure all dry ingredients are incorporated. There should not be any streaks of flour in the batter.
Fold in egg white meringue in three batches gently until just incorporated. Scrape the sides and bottom of the bowl to ensure the meringue is incorporated.
(F) Make the Chocolate Batter: If you have weighed the mixing bowl previously, scoop 30% (by weight) of the vanilla batter into the dissolved cocoa paste. If not, simply eye-ball slightly less than 1/3 of the batter. Fold until the cocoa paste is incorporated into the batter.
(G) Layer the Batter: Scoop 2 scoops of vanilla batter into the baking tin, followed by 1 scoop of chocolate batter. Repeat, ending with vanilla batter. Gently shake the tin to even out the batter. Dip a butter knife into the batter. Without removing the knife, swirl the batter a few times to create a marbled top. Remove the knife.
(H) Bake the Cake: Bake the cake for 30 to 40 minutes until a skewer inserted into the centre of the bake comes out clean. Let the cake cool in the tin for 15 minutes. Invert the cake out of the tin, then invert again and place on a wire rack (i.e. the pretty side is facing up). Let the cake cool completely before serving.
(I) Storage: Store the cake in an air-tight container in the refrigerator for 3 to 5 days. Let the cake come to room temperature or warm it in the microwave (I do 20 seconds at 800W) before serving.