Happy 3s and Buttercream
The brief
Normal sudoku rules apply.
Every cell in the grid belongs to a square / rectangular region, to be located. Each region contains a cupcake. The digit on a cupcake indicates its region's size. Digits may not repeat in regions.
Digits separated by an X sum to 10.
Every region has four DIFFERENT corner cells, none of which are allowed to contain a 3.