Bündner Nusstorte (Sealed Caramel Walnut Tart)

Variations

• It has two names: Bündner Nusstorte after the canton (Graubünden) and Engadiner Nusstorte after the valley are the same tart. It was created in the Engadine in the early 20th century and spread across the canton; it is now sold as a souvenir all over Switzerland. Wikimedia Commons files it under the Engadine name.
• It began as a keeping cake: sealed shut and low in moisture, it lasts for weeks in an airtight tin. That is a mountain-village idea — food made to be kept through a winter — and it is why it is still sold as a gift.
• The nuts: walnut is standard, but hazelnut or pecan work. Either way they must be toasted first.
• Chocolate decoration: confectioners set half-dipped walnut halves on top. That is what the photograph on this page shows, and the quantities are in the ingredient list.
• Against dobos torte — same sugar, opposite aims: that one drives every trace of water out of the caramel so it sets into a sheet; this one pours cream in so it never sets at all. Two roads out of the same pan.
• Against tanghulu: driven to 155°C and set like glass — the third point on that axis.
• Against apple pie: both are sealed double-crust pies with steam holes cut in the lid. What differs is the filling — apples release their own liquid, while caramel is already thick and looking for a way out.
• Against scones: the same attitude to pastry — cold butter, minimal working, and keeping the gluten out of it.

Tips

• Never stir a dry caramel. Stirring melting sugar seeds crystals, and one crystal propagates until the whole pan sets grainy. Swirl the pan instead.
• Warm the cream. Always. Nearly every failure in this recipe is here. Cold cream hitting caramel at around 180°C drops the temperature instantly, the sugar sets, and that lump takes a long time to melt back.
• Keep your face away. The water in the cream flashes the moment it touches the caramel and it boils up hard. Use a long-handled spatula and a pan bigger than you think you need.
• Why it stays soft: sugar melted alone hardens like glass as it cools. Cream introduces water and fat that get between the sugar molecules and stop them locking back together. The honey does the same job — invert sugar interferes with crystallisation (what sugar does).
• Cool the filling completely. Even lukewarm caramel melts the butter in the pastry base, makes it soggy, and leaks through it in the oven.
• Do not knead the pastry. Add the egg to the sandy crumbs and stop the moment it holds together. Worked until smooth, it bakes hard.
• Seal the edges properly. Escaped caramel sets on the base of the tin and glues the tart to it. Lining the tin gives you a way back from that.
• Do not forget the steam holes. Caramel boiling inside a sealed pie has nowhere to send its steam, so the lid domes and cracks. Three or four fork holes is enough.
• Cool it in the tin. Turned out warm, the base breaks. Cold, the caramel is firm and it cuts cleanly.
• It is better after two days. The pastry takes up a little moisture from the filling and softens, and the flavours settle. That is also why it travels as a gift.