Cut the peeled rhubarb into small cubes approx. 8 mm, place in a saucepan with sugar and cinnamon and let caramelize for approx. 3 - 4 min. It should become soft, but not disintegrate. Then let cool down completely.
Beat the butter and both sugar sorters whitish-creamy. Stir in egg and vanilla extract. Mix the flour, baking soda, baking powder and salt, sieve into the butter mixture and stir well. Fold in the cooled rhubarb including some formed stock. It mixes with the dough, is only faintly visible in some places, but gives the cookies a fruity taste, makes them loose on the inside and a little caramel on the outside ....... (and nobody noticed that it was Rhubarb is ... ;-))) Cover the bowl with the very soft dough and place it in the refrigerator for 1 hour to set.
The above number of people refers to a serving of 20 pieces.
With convection (180 °) and 2 retractable trays (which I unfortunately do not have) the baking time is shortened.