Siberia (シベリア, castella with yōkan)

Ingredients

Equipment Needed

• •Wooden castella frame (kasutera wakugata), or an 8-inch square cake pan lined high with parchment
• •Yōkan mold (nagashikan), or the same square pan lined with parchment
• •Hand mixer or stand mixer
• •Fine-mesh sieve
• •Offset spatula
• •Instant-read thermometer, helpful but not required
• •Small flat board or pan for light weighting

Instructions

1. 1

   Line the pan

   Heat the oven to 325°F (160°C). Line an 8-inch square pan with parchment, letting the paper rise above the sides. Siberia depends on straight layers, and a well-lined pan gives you clean edges without wrestling the cake out later.

2. 2

   Warm the honey

   Stir the honey into the warm water until loose and pourable. Honey gives castella its quiet fragrance and helps the crumb stay moist, but it must be thinned first so it disappears evenly into the egg foam.

3. 3

   Whip the eggs

   Put the eggs and castella sugar in a mixer bowl and set it over a bowl of warm water for a minute, whisking until the mixture feels just warm to the touch. Beat on high until pale, thick, and ribbony, 7 to 9 minutes. Warm eggs hold more air, and that air is the lift. There is no butter or baking powder here to rescue a lazy foam.

   Lift the whisk. The batter should fall in a thick ribbon and sit on the surface for a moment before sinking. That pause tells you the foam is strong enough.
4. 4

   Fold the flour

   Beat in the honey mixture on low speed, then sift the bread flour over the batter in three additions, folding after each one until no dry streaks remain. Bread flour gives castella its slight chew and steady body. Fold cleanly, scraping from the bottom, because hidden pockets of flour bake into pale seams.

5. 5

   Bake the castella

   Pour the batter into the lined pan and run a skewer through it in a few slow lines to break large bubbles. Bake for 30 to 35 minutes, until the top is deep golden and springs back when pressed lightly. Drop the pan once from a few inches above the counter after baking. That little shock helps the crumb settle evenly instead of shrinking in a sulky crater.

6. 6

   Cool and slice

   Lift the castella out, peel away the side paper, and wrap it loosely while it cools. When fully cool, trim the edges and slice the cake horizontally into two even slabs. A wrapped rest keeps the sponge moist, and a cool cake cuts cleanly. Try to hurry this and the knife will drag the crumb.

7. 7

   Boil the kanten

   Line the same pan again with fresh parchment. In a saucepan, whisk the powdered kanten into the 250ml water and bring it to a full boil. Boil for 2 minutes, whisking. Kanten must be properly boiled to dissolve; if it only warms, the yōkan may set weakly or weep at the edges.

8. 8

   Cook the yōkan

   Add the yōkan sugar and stir until dissolved. Add the koshian, salt, and mizuame if using, then cook over medium-low heat for 5 to 7 minutes, stirring constantly, until the mixture is glossy, thick, and falls from the spatula in heavy folds. This short cooking concentrates the water so the finished yōkan cuts as a firm slab, not a soft paste.

   Use smooth koshian for the old bakery look. Tsubuan, the chunky paste, is good food, but it changes the clean face of Siberia.
9. 9

   Temper and assemble

   Set one castella slab in the lined pan, cut side up. Stir the yōkan off the heat until it thickens slightly and looks glossy, still pourable but no longer hot enough to soak the cake, about 130°F to 140°F (55°C to 60°C). Pour it over the castella and level it quickly, then set the second slab on top, cut side down. This is the one detail that decides the dish: warm enough to bond, cool enough not to flood the crumb.

10. 10

    Weight and set

    Cover the top with parchment and set a small flat board or another pan over it with a light weight. Chill for at least 3 hours, until the yōkan is firm all the way through. The weight keeps the layers in contact while the kanten sets, so the slices hold together instead of politely falling apart.

11. 11

    Cut clean slices

    Lift the slab from the pan and trim the edges. Cut into 9 squares or 12 narrow rectangles with a long knife dipped in hot water and wiped dry between cuts. Clean steel gives you clean faces, and Siberia is a sweet that shows every hesitation. Serve small pieces with tea and leave the plate room.

Chef Tips

• •Use kanten, not gelatin. Gelatin gives a softer wobble and a different mouthfeel; kanten gives yōkan its clean bite and tidy edge. Here, that edge is not decoration. It is the dish.
• •A bought plain castella is a sensible stand-in if you want to focus on the yōkan and assembly. Choose one with a close crumb and no cream, glaze, or flavoring. The sandwich should taste of honey cake and azuki, nothing hidden.
• •If you cannot find mizuame, leave it out or use a small spoonful of light corn syrup. It adds gloss and smoothness, but the kanten and koshian do the real work.
• •Cut the finished slab cold, then let the pieces sit 15 minutes before serving. Cold yōkan cuts neatly, but the castella tastes softer and more fragrant once the chill relaxes.

Advance Preparation

• •The castella can be baked a day ahead, wrapped once cool, and kept at room temperature. It slices more cleanly after a rest.
• •The assembled Siberia keeps for 3 days refrigerated, well wrapped. Bring it back toward cool room temperature before serving so the honey in the cake can be tasted.
• •Do not freeze it. The yōkan may turn grainy and the castella loses the close, moist crumb you worked for.

Frequently Asked Questions

Nutrition Information

1 serving (about 115g)