Magic Chocolate Custard Cake

I've wanted to try this recipe for a long time. I've seen many pictures of magic custard cakes but this is the first chocolate on I've seen. The magic happens in the oven when the cake batter separates into 3 layers- a cake layer, a more dense cake layer and a custard layer. I was very concerned while making the batter that this cake would not turn out. The batter was runny and curdled. After baking, the cake was slightly jiggly in the center and had a small crack in the middle. After refrigerating overnight it was the perfect blend of creamy, chocolatey, moist, cakey goodness.

Magic Chocolate Custard Cake

Inspired by White on Rice

1/2 cup  (113 g) unsalted butter, melted and cooled

2 1/2 cups (600ml) whole milk, lukewarm

1 cup (115g) all purpose flour

1/2 cup (43g) cocoa powder

4 eggs, separated

6 drops distilled white vinegar

1 3/4 cups (210g) confectioner's sugar

2 Tablespoons (30ml) espresso,  lukewarm

1 teaspoon (5ml) vanilla extract

extra confectioner's sugar for dusting 

Directions:

1. Preheat the oven to 325°F. Lightly butter a glass 8"x8" baking dish.
2. In a medium bowl, whisk flour and cocoa powder.
3. In a stand mixer with whisk attachment, whip the egg whites with vinegar to stiff peaks.
4. In a large bowl, beat the egg yolks and sugar. Then beat in the melted butter, espresso, and vanilla extract until evenly incorporated.
5. Stir in the flour/cocoa into the batter until incorporated. Gently whisk in the milk until well mixed.
6. Fold in the egg whites, 1/3 at a time. Repeat until all of the egg whites are folded in. The batter will look curdled and runny.
7. Pour the batter into pan and bake for 50-60 minutes until slightly jiggly in the center (like a cheesecake).
8. Cool cake on wire rack. Sprinkle with confectioner's sugar. If not serving soon, refrigerate.