In a small saucepan, combine the egg yolk, 1 tablespoon honey, vanilla extract, and cornstarch, and whisk together until well-mixed. Add the milk and whisk again to combine. Bring the mixture to a boil over medium heat and then lower to a simmer. Let the mixture simmer gently, whisking occasionally to keep the egg yolk from cooking on the bottom of the pan, until it has the consistency of dense, homogenous cream, almost as thick as ice cream, about 5 minutes. Remove from the heat and let cool.
Place the rhubarb slices in a large heavy-bottomed pot and cover with the remaining honey. Cook gently over low-medium heat until the rhubarb is tender, about 10 minutes. Let cool.
Fold the rhubarb into the milk mixture and gently fold in the whipped cream. Divide the rhubarb cream evenly among 4 glass serving bowls or glasses and serve immediately.