Butter pooling under your croissants means the layers broke, and you can find where

These are listed in the order worth acting on, which with butter leaking from laminated pastry is not the order they are usually presented in.

Normal versus failed

Some butter always renders out of a croissant during baking, and that fat fries the base into the crisp, slightly caramelised underside you want. A failure looks different: a substantial pool of clear yellow fat, a greasy pastry that has collapsed slightly, and a bready rather than honeycombed interior. The test is the crumb, since a properly laminated croissant shows an open, irregular network of thin walls and visible strands.

A dense, uniform interior means the butter and dough merged before the steam could push them apart in the oven. Establishing which of the two you have is the first step, because a lightly fried base is a sign of success rather than failure.

Proving too warm

This is by far the most common cause, and it happens because most warm spots people use are hotter than the butter can survive. Butter begins softening well below body temperature and starts flowing somewhere around 30 to 32C depending on its composition.

Weighed out, a shaped croissant proved in an airing cupboard or a sunny window at 30C will have lost its structure before it ever reaches the oven. Prove at 24 to 26C instead, and expect two to three hours rather than the forty five minutes a warmer spot would take. You should be able to touch the croissant without the surface feeling greasy at any point during the prove.

Torn butter during rolling

If the butter block cracks or breaks through the dough during a turn, that break becomes a channel through which fat escapes in the oven. Butter that is too cold shatters into shards, leaving gaps; butter that is too warm smears into the dough and stops being a separate layer. The workable window is roughly 15 to 18C for both the butter and the dough, and they need to be at similar firmness.

At oven temperature, look at the cut edge of the dough before shaping, since a clean set of parallel striations means the sheets survived the rolling. Where butter has broken through, flour the spot lightly, chill immediately for twenty minutes, and continue rather than starting over.

Skipping the rests

Rolling stretches the gluten, and a stretched dough contracts as soon as you stop, dragging the butter sheets and tearing them. Thirty to forty minutes in the fridge between turns relaxes the gluten and firms the butter at the same time, which is why the rest does double duty.

Rushing through three turns in twenty minutes is the classic route to a dough that fights back and shears its own layers. The freezer is not a shortcut, since it firms the outside faster than the centre and gives you a hard shell over soft butter. If the dough resists and springs back as you roll, stop and chill rather than pressing harder.

Rolling too thin at the end

The final sheet has to be thin enough to shape but thick enough that each butter layer is still continuous. Rolling a well laminated block down to two millimetres often tears sheets that would have survived at three or four. Edges suffer first, because the layers wrap around there and are already thinner than in the middle of the block.

At oven temperature, trimming the edges before cutting triangles removes the weakest material and improves the consistency of the batch. Cut with a sharp downward press rather than dragging a blunt knife, which seals the layers shut at the cut face.

The oven stage

A cool oven lets the butter melt and run before the steam has generated enough pressure to separate the layers. Start hot, typically 200 to 220C, so the water in the dough flashes to steam quickly and does its work early.

By the second rise, a thick egg wash glues the layers together at the edges, restricting the rise and forcing fat out sideways instead. If the croissants look correct going in and leak anyway, the oven temperature is the remaining variable to check. An oven thermometer resolves this faster than any adjustment to the dough.

Everything above, in order of what to do first

1. Normal versus failed. Some butter always renders out of a croissant during baking, and that fat fries the base into the crisp, slightly caramelised underside you want.
2. Proving too warm. This is by far the most common cause, and it happens because most warm spots people use are hotter than the butter can survive.
3. Torn butter during rolling. If the butter block cracks or breaks through the dough during a turn, that break becomes a channel through which fat escapes in the oven.
4. Skipping the rests. Rolling stretches the gluten, and a stretched dough contracts as soon as you stop, dragging the butter sheets and tearing them.
5. Rolling too thin at the end. The final sheet has to be thin enough to shape but thick enough that each butter layer is still continuous.
6. The oven stage. A cool oven lets the butter melt and run before the steam has generated enough pressure to separate the layers.

The takeaway

Check the prove temperature first, the cut edge second, and the oven third.

Bake it badly once on purpose. The failure teaches more than the photograph.