Make the plum puree:

Quarter the plums and discard the pits. Combine the plums, 2 tablespoons sugar, and ¼ cup of water together in a medium-sized saucepan. Bring to a boil over medium-high heat, reduce the heat, and simmer for 10 to 15 minutes. Pass the cooked plums and their cooking liquid through a fine-mesh strainer, using the back of a ladle or rubber spatula to push as much of the fruit through as possible. Discard the skins. Set the puree aside to cool.

Caramelize the sugar (see the Note on cooking caramel successfully):

In a covered heavy-bottomed saucepan, combine the remaining sugar, cream of tartar, and 3 tablespoons water. Cover and bring to a boil over medium-high heat. After 1 minute of rapid boiling, uncover the pot. Cook the syrup until the sugar caramelizes and reaches a light golden brown color, about 5 minutes. Remove from the heat, stand back, and carefully add the plum puree. The caramel will erupt with bubbles and steam. Place the pot back on the stove over medium heat and whisk the plum puree until the caramel is completely dissolved and smooth. Remove from the heat and cool.

Serving Suggestions:

Serve this sauce cold or warm. Or serve it at room temperature with the Roasted Apple or Pear Beignets, Plum and Almond Tart, or Vanilla Panna Cotta.