Gingerbread Stairs Inner Corners were implemented during the May 2017 launch update, together with the other Roof and Slope Corner blocks.
Gingerbread Stairs Corners are created by putting gingerbread stairs blocks into the processor.
Gingerbread blocks and recipes can be found in randomly spawning Holiday Gift-boxes during the Festive Season event around Christmas. There are also player-created adventures that offer these recipes.
The stairs corner blocks can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.