Butter Building (バタービルディング) is a presumed stage in Super Smash Flash 2. The stage was initially leaked via CR Drake's livestream. Besides a placeholder stage icon, there is nothing else to show for the stage at this point in time. However, according to former developer RoyMaster4, the stage would be a vertically scrolling level in the vein of Melee's Icicle Mountain and Brawl's Rumble Falls. This stage may or may not be a future addition in the game.
Screengrab from Ya Dad's livestream showing Butter Building on the third row.