Step 1

Gather all the ingredients. Preheat the oven to 350ºF (180ºC). For a convection oven, reduce cooking temperature by 25ºF (15ºC).

Step 2

Line a 6-inch (15-cm) round cake pan with parchment paper (I use this 6x3 inch pan). I spray oil on the pan first so the parchment paper will stick to the pan. Note: You will need to double the recipe for an 8-inch or 9-inch cake pan, and triple the recipe for a 10-inch or 11-inch cake pan. Please note that the thickness of the cake will be varied.

Step 3

If you're using the same brand of a 4-oz white chocolate bar, remove one square block of chocolate so it'll be 3.5 oz (100 g). Or use a kitchen scale to measure the chocolate precisely. Break or chop up the chocolate into smaller pieces and put them in a heatproof large metal or glass mixing bowl (make sure the bowl is moisture or grease-free).

Step 4

Cut 3 Tbsp butter into small cubes and add to the bowl.

Step 5

To set up a double boiler, bring 2 inches of water in a pot to a simmer (gentle boil) and then place the bowl with your ingredients inside over the pot. You want it to fit snugly without touching the water. Melt the chopped white chocolate and butter in the double boiler.

Step 6

With a silicone spatula, mix and let the chocolate and butter melt completely.

Step 7

Carefully remove the bowl from the double boiler. Make sure the steam or water doesn't get into the chocolate mixture (it will seize the chocolate). Add 3 Tbsp matcha and mix them all together completely.

Step 8

Gradually add ¼ cup milk and mix well together.

Step 9

Separate 3 eggs into egg whites and yolks. Keep the egg whites in the refrigerator or freezer until you use them (In Japan, instead of cream of tartar, we commonly use cold, sometimes partially frozen, egg whites to make meringue).

Step 10

Add 3 Tbsp sugar to egg yolks and whisk until the mixture looks pale yellow.

Step 11

Add the matcha chocolate mixture to the egg yolk mixture and mix the batter well.

Step 12

Sift 3 Tbsp cake flour over the cake batter.

Step 13

Gently whisk the cake batter until just combined.

Step 14

Add ⅛ tsp salt and mix it all together.

Step 15

Add 3 egg whites into another clean large bowl. Beat the egg whites with an electric mixer.

Step 16

When bubbly, gradually add 3 Tbsp sugar while beating the egg whites.

Step 17

Beat until stiff peaks form. You've reached a stiff peak stage when the egg whites are shiny, and the tips can stand straight up even when turned upside down. Tip: If you've never beaten egg whites before, slow down the whisking when it's getting close to stiff peaks. Keep checking so you won't overbeat. Troubleshoot: The earliest signs of overbeating are little granules on the side of the bowl and decreased volume. If it overbeaten, the structure of the egg whites will break. Try beating in another egg white to help recover the mix or start over again.

Step 18

Fold a third of the egg whites into the batter. With the whisk, stir it around to lighten the batter and loosen it up for additional foam.

Step 19

Carefully add the next third of the egg whites on top of the batter and, using a rubber spatula, turn the egg whites into the batter. The important part here is not to stir. Instead, use a flipping motion, literally folding the egg whites into the batter. Finally, add the remaining whites and repeat the folding motion until the foam is just incorporated. Don't overdo it, or you'll end up releasing all the air you've carefully whipped into the egg whites.

Step 20

Transfer the cake batter into the prepared cake pan. Smooth out the surface with the spatula.

Step 21

Bake at 350ºF (180ºC) for 15 minutes and reduce the temperature to 325ºF (160ºC) and bake for 20 minutes. The cake should be wobbly and the inside should be woozy (therefore, no need to insert a skewer to check). It's best to let cool completely, about 4 hours because it's impossible to cut the cake. You can keep the cake in the refrigerator overnight.

Step 22

I highly recommend microwave a slice for 5 seconds (from the fridge) to warm up the cake. Dust powdered sugar on top, if you like, and serve. The cake literally melts in your mouth!

Step 23

You can keep the cake in an airtight container and store in the refrigerator for 3 days and in the freezer for a month.