Mayan Roof Inner Corners were implemented during the May 2017 launch update, together with the other Roof and Slope Corner blocks.
Inner Corners are created by putting Hidden Temple Roof blocks into the processor, giving you 1 inner corner block per roof block.
Hidden Temple Roof blocks are part of the Hidden Temple blueprint and block-set. You can buy either the Hidden Temple super bundle, which gives you the recipes, or the Hidden Temple pack, which gives you a sample of each block, but not the recipes.
The roof blocks can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.