Place the plums, maple syrup, lemon juice, water, and cinnamon stick in a saucepan on medium-low heat. Once the plum mixture reaches a simmer, stir and reduce the heat. Remove from the heat once the plums have become tender yet still maintain their shape, about 15 minutes. Slightly cool the plums and stir in 1 cup toasted whole walnuts into plum mixture and remove the cinnamon stick. Chop the remaining 1/4 cup toasted walnuts. To serve, divide the yogurt among the bowls and spoon Maple Plum Compote over and top with the chopped walnuts.