Location

It is in the eastern edge of the mouse area. You'll need to make it to the crossroads and take the path to the northern room where there are a few mouse head ghosts, a Switch, some Toggle Blocks and a hidden path to a room on the right behind vines.

You can use the B. Wand to create a bubble under you to get enough height to get into the room to the right.

Use the Lantern to safely go through the mouse ghosts and reach the chest.