From Magnus Nilsson's The Nordic Baking Book: "Kladdkaka is a relatively recent addition to Swedish cake culture. Its origins are a bit unclear and the accounts on where it came from are as colorful as they are conflicting. One thing is certain though: there was no kladdkaka before the mid-1970s. One can also assume without going out too much on a limb that the unleavened, very gooey and soft cake of today is the result of naturally occurring cake evolution. Someone had a really good recipe for chocolate cake, perhaps a brownie one, or why not something more central European in style? The same person, in the heat of the moment, forgets to add baking powder only to realize his or her mistake halfway through the cooking process. They then remove the undercooked cake from the oven and they are astounded by its deliciousness. Those character traits that were liked with the first failed cake have then, through generations of cakes, been reinforced, whilst those that weren’t liked have been changed to improve it. Kladdkaka only exists in Sweden and during its brief existence it has gained a huge following. It is actually the most common recipe search from Swedish websites. If you type it into your Internet search engine you will get well over 650,000 hits.
There are as many conflicting ideas on how to eat this cake as there are recipes for it. Some like it warm with ice cream, some like it at room temperature with nothing, and I like it as my wife will tell you to eat it: cold from the refrigerator with whipped cream on the side." —Ella Quittner
Preheat the oven to 175ºC/345ºF/Gas Mark 4. Butter a 24-cm/9 1/2-inch springform cake pan and coat with breadcrumbs.
Melt the butter in a medium pan over a low heat. Add the chocolate and keep melting everything together while stirring. Remove the pan from the heat and add the rest of the ingredients while you keep stirring until well combined.
Scrape the batter into the prepared cake pan, smooth the surface and bake for 15 to 20 minutes. It should be sticky on the inside. If the cake gets too dark, cover it with aluminum foil for the last 5 minutes.