A baked Alaska survives the oven because foam conducts heat badly

The trick is conductivity and not speed alone

The usual explanation for a baked Alaska is that the oven is very hot and the bake is very quick, so the heat has no time to get through. That is half of it, and the less interesting half. The reason the timing works at all is that the meringue between the oven and the ice cream is an extremely poor conductor of heat, so the temperature gradient across it is enormous — scorching on the outside and still below freezing a couple of centimetres in.

Still air conducts heat far worse than water, ice, sugar or protein do. A stiff meringue is mostly air by volume, held in cells small enough that convection currents cannot form inside them, which is the same arrangement that makes wool, expanded foam and double glazing work. The protein and sugar aren’t there to insulate. They are there to hold the air still, and holding air still is what insulation consists of.

The interior is also buying time by melting

There is a second mechanism working in the dessert’s favour, and it is one of the more useful facts in a frozen kitchen. Melting ice absorbs a large amount of energy without any rise in temperature at all — the latent heat of fusion of water is around 334 kilojoules per kilogram, which is roughly the energy needed to heat the same mass of liquid water by eighty degrees. Any heat that does get through the meringue is therefore spent melting a thin skin of ice rather than warming the whole thing.

So the interior is defended twice over. The foam slows the arrival of energy, and the ice absorbs what arrives without changing temperature until it has entirely melted. This is why the failure mode is a soft outer film rather than a puddle, and why a dessert that has been in the oven slightly too long is disappointing rather than destroyed.

The assembly rules follow directly from all this

Every instruction in a baked Alaska recipe is now readable. The ice cream must be frozen genuinely hard beforehand, because the process is a race that starts wherever you leave it. The meringue must be thick, since insulation scales with thickness and a thin covering is barely a covering at all. And it must be sealed right down to the plate or the base, because any gap is a direct path for hot air, and hot air moving is a far better heat transfer mechanism than the foam it is bypassing.

The sponge or biscuit base is doing the same job from underneath. It is porous, dry and full of air, so it insulates the ice cream from the dish and from conducted heat below. A dense, moist base conducts better and works less well, which is the practical argument for a light sponge over anything rich. None of this is decoration, and none of it is optional.

A blowtorch and an oven are not the same problem

Browning a meringue with a torch changes the terms considerably. A flame delivers intense heat to a very small area for a very short time, so the total energy entering the dessert is small even though the surface temperature is extremely high. That suits a delicate assembly and it is why a torched meringue can sit on something barely frozen without incident.

An oven is a bulk method. The whole surface is heated for the whole duration, and the energy arriving is far greater even though the temperature is lower, so an oven-baked version needs the full thickness of foam and the hard-frozen interior in a way a torched one doesn’t. Neither approach is better. They put different demands on the construction, and a recipe written for one does not transfer cleanly to the other.

The same principle turns up in several other desserts

Fried ice cream works on exactly the same logic, with a coating of crumbs or batter standing in for the meringue and a very short immersion standing in for the short bake. The coating is porous, the oil is hot, and the interior has its latent heat to fall back on. So does the omelette norvegienne, which is the same dessert under a different name and often flamed rather than baked.

It is a good demonstration of a general point about frozen desserts, which is that they resist heat much better than people assume. A large block of ice cream is thermally sluggish, its surface can be scorched while its centre is unaffected, and the practical enemy in every case isn’t the oven but the time spent standing about at room temperature before or afterwards. That is where the damage actually happens.