Taiwanese Castella Cake

Prepare the pan and oven. Preheat the oven to 300°F (150°C) and line all the edges of an 8x8x3 inch pan with parchment paper with the parchment paper 1 inch higher than the pan. If your pan is not completely sealed along the corners, wrap the outside of the pan with aluminum foil. Heat hot water to about 175°F or (79°C) for the water bath.

Sift the cake flour and salt. In a large mixing bowl, sift the cake flour and salt to remove any clumps.

Warm butter and milk. In a small saucepan, add the butter and milk over medium heat. Heat until the butter is just melted and steamy about 120°F (49°C). Do not bring to a simmer or boil.

Combine. Combine the warmed butter and milk with the sifted cake flour. Whisk until combined.

Add egg yolks one at a time and vanilla. Add the egg yolks one at a time, whisking to incorporate each yolk before adding the next one. Whisk until thoroughly combined. Then add the vanilla extract and mix to combine.

Make the meringue. In a large mixing bowl or in the bowl of your stand mixer with the whisk attachment, wipe the bowl and the whisk with a thin layer of white vinegar. Then beat the egg whites and cream of tartar on low speed until foamy. While the mixer is beating, slowly pour 1/3 of the sugar and mix to incorporate at medium speed. Repeat with the remaining 2 batches of sugar until the meringue has reached soft peaks. You will know the meringue is done when you lift the whisk and the meringue tips flop over and do not hold a stiff peak.

Combine. Combine 1/3 of the meringue into the egg yolk mixture and fold by using a spatula and lifting the two mixtures and turning them over until combined. Repeat two more times with the remaining meringue. Continue folding until uniform in color.

Pour the batter. Pour the batter into the prepared cake pan from about 12 inches from the pan. Then tap the pan on the counter to remove any bubbles. Use a toothpick to remove any remaining bubbles.

Place in a water bath. Place the cake pan in a larger baking pan and then pour in the hot water (175°F or 79°C) to about 1 inch to 1.5 inch up the side of the square pan.

Bake. On the lower 1/3 rack in the oven, bake the castella cake for about 65-70 minutes, or until the cake is golden brown at the top and a toothpick inserted in the center comes out clean.

Serve. Let the Taiwanese castella cake sit for 2 minutes before removing it from the pan, peeling the parchment paper, and slicing. The cake will slightly deflate as it cools.