☐ Unbreakable Yellow Block in Bowser Level of Paper Mario 2

I liked the game Paper Mario: The Thousand-Year Door. However I cannot figure out how to break the yellow block in the last Bowser level (accessible in 3-1 and 3-2). It is surrounded by unbreakable blocks.

Huge bowser standing near unbreakable yellow block in “Paper Mario 2”

Small bowser standing next to unbreakable yellow block in “Paper Mario 2”

I’ve tried all the different sizes that Bowser can be in that level, and none seem to work. If he’s huge, he still cannot break the grey blocks and his breath doesn’t reach the yellow block, and if he is small enough to fit under there, his fire is insufficient to break it and there is not enough room for him to pound-jump on it.

How do you break the yellow block? What is in there?