And why are they such a problem with cannelés? So asks reader Ronnie, and it’s a question well worth answering. After all, almost all cakes — both large and small — are filled with bubbles. They don’t explode, why do cannelés? Well Ronnie, the problem is the shape of the molds. They’re tall and narrow rather than broad and flat, the shape of most cake and cookie molds. And that’s a problem when it comes to expanding gas and steam. Normally the expanding gasses in, say, a financier can escape out the top and to a lesser extend the sides, which together make up a considerable surface area.
With cannelés you have far less exposed surface relative to the deep sides of the mold, so those metal sides end up holding in the rise like a girdle. As the batter gets hotter the gasses have nowhere to go but up, and when they do they blow the top off these little cakes. Most people are drawn to the cannelé because of its compelling presentation: a tall-and-narrow shape. That’s very unusual in the world of baking pastry, where most tall things are actually stacked up collections of flat things. Making them you realize that there’s good reason that most baked things are flat, and that we bake batters and doughs on sheets instead of in bottles.
Indeed, virtually all baking formulas are written with the assumption that the vast majority of the leavening power needed for bread or cake X will be lost out the sides and top of the whatever-it-is. Cannelés are unusually, viciously efficient. They use almost every last bit of the leavening power they’re given, and we all pay dearly as a result.