Kakuro (Double) by Grant Fikes
or solve online (using our beta test of Penpa-Edit tools; note that the dotted circles are just for answer entry and are where the number should be placed in the 2×2 cell. An alternate option suggested in the comments below with centralized digit entry is here.)
Theme: Circular Reasoning
Author/Opus: This is the 342nd puzzle from our contributing puzzlemaster Grant Fikes.
Rules: Variation of Kakuro. The gray (2×2) cells are to be filled by a single digit which sits in multiple rows and columns. No digits can repeat within an entry, regardless of if it is in a small or large cell.
Difficulty: 1.5 stars
Time Standards (highlight to view): Grandmaster = 0:50, Master = 1:45, Expert = 3:30
Solution: PDF; a solution video is also available here.
Note: Follow this link for classic Kakuro and this link for Kakuro variations. More Kakuro puzzles can be found in Kakuro and Variations by Serkan Yürekli and in The Art of Puzzles 2.
I found this easier to solve with “Border” set to on and using Number instead of Sudoku, to place numbers directly into the center of the 2×2 cells.
We tested both modes and each had pros and cons, as the default UI for Sudoku is a little better but centered values are easier to read. I decided not to post two penpa options but we can add a Number/centered version here as well.