From the Super Mario Wiki, the Mario encyclopedia
Doughnut blocks, or donut blocks, are a kind of block Wario encounters during his adventures in the Wario Land series. Their appearance differs between the various installments, but they all share the property that they are not breakable by any of Wario's standard attacks. Naturally, they are placed in specific places to prevent access to certain areas. To break them regardless, Wario has to assume his fat reaction. The additional weight he has in this form allows him to break doughnut blocks by jumping on top of them.
Wario Land series
Doughnut blocks first appear in Wario Land II aboard the SS Tea Cup, and are a common obstacle throughout the game afterwards. They appear as brown blocks with some cracks in them. A Cook can usually be found near them, so that Wario is able to break them. The doughnut blocks also reappear in the sequel Wario Land 3, where they change their appearance, but retain their purpose. In this game, they are bright pink in color and sport a donut symbol on them. Like in the previous game, enemies invoking the fat reaction can be found near them, namely Doughnuteer or Appleby.