Place all compote ingredients in a saucepan & simmer covered 3-5 minutes. Remove cover & stir, continue stirring for another 5 minutes making sure to avoid scorching the pan. Set aside to cool.
Cookies
Preheat oven to 350 F. Line a large baking sheet with parchment paper.
In a medium bowl, place softened butter, brown sugar, 1 tsp ground cardamom, salt & vanilla. Using a hand mixer, beat until light color & fluffy, scraping down the bowl as needed with a spatula, about 2 minutes.
Add egg & beat until combined. Fold in flour until just combined.
In a shallow bowl, place the sugar, cinnamon & remaining 1/2 tsp cardamom. Mix well to combine.
Scoop the dough out by level Tbsp & roll each into a ball. Roll each ball in the cardamom sugar & place on the baking sheets, spacing them evenly apart. Make an indentation about 1/2-inch deep with your thumb into each ball.
Bake 7 minutes. Rotate the baking sheet from front to back. Bake until the cookies take on a tiny bit of color (they don’t really brown) & are dry to the touch, 5-7 minutes more. If indentations have lost their shape, remake them with the handle end of a wooden spoon. Let cookies cool completely on the baking sheet.
Fill each cookie indentation with the cooled rhubarb compote. Let sit until the filling sets slightly, about 20 minutes, before serving.