Attack Piece Block
From the Super Mario Wiki
Attack Piece Blocks are blocks from Mario & Luigi: Bowser's Inside Story with the letter "A" on them. They resemble pieces of a puzzle joined together to make a block. Attack Piece Blocks are scattered throughout Bowser's body and in places outside Bowser such as Dimble Wood. Blocks inside Bowser's body look two-dimensional, while blocks in the 3D outside world are 3D. There are ten Attack Piece Blocks in almost every area; they will give Mario and Luigi an Attack Piece if struck. Hitting all ten will give the bros. a new Special Attack. Sometimes Mario and Luigi can find several of these blocks all at once. Once in a while they can get all ten at once.
Attack Piece Blocks return in Mario & Luigi: Dream Team in an identical fashion, except instead of Special Attacks, they produce Bros. Attacks when completed, exclusive to Mario or Luigi in the Real World, and also produce Luiginary Attacks for Mario and Dreamy Luigi in the Dream World when found and completed there.